Fix breaking change introduced in #204

context: #204 (comment)

`Kernel.#Array` calls not only `Object#to_ary` but also `Object#to_a` but some classes which represent cluster may implement `to_a`.

https://api.rubyonrails.org/classes/Array.html#method-c-wrap

I think that's why ActiveSupport has `Array.wrap`.

https://api.rubyonrails.org/classes/Array.html#method-c-wrap

I confirmed there's no performance regression via the following snippet:

```ruby
require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  gem "benchmark-ips", "2.7.2"
  gem "cancancan", "2.3.0"
  gem "allocation_tracer", "0.6.3"
end

module CanCan
  class Rule
    def initialize(base_behavior, action, subject, conditions, block, __flag__:)
      both_block_and_hash_error = 'You are not able to supply a block with a hash of conditions in '\
                                  "#{action} #{subject} ability. Use either one."
      raise Error, both_block_and_hash_error if conditions.is_a?(Hash) && block

      @match_all = action.nil? && subject.nil?
      @base_behavior = base_behavior
      case __flag__
      when :array
        @actions = Array(action)
        @subjects = Array(subject)
      when :flatten
        @actions = [action].flatten
        @subjects = [subject].flatten
      when :wrap
        @actions = wrap(action)
        @subjects = wrap(subject)
      else
        raise
      end
      @conditions = conditions || {}
      @block = block
    end

    def wrap(object)
      if object.nil?
        []
      elsif object.respond_to?(:to_ary)
        object.to_ary || [object]
      else
        [object]
      end
    end
  end
end

def memprof(name)
  GC.disable
  before = `ps -o rss -p #{Process.pid}`.split("\n").last.to_i
  ObjectSpace::AllocationTracer.trace do
    10_000.times do
      yield
    end
  end
  after = `ps -o rss -p #{Process.pid}`.split("\n").last.to_i

  puts "[#{name}] Mem diff: #{(after - before).to_f/1024.0} MB"
  GC.enable
  GC.start
end

puts "Ruby #{RUBY_VERSION}"

# I think memprof outputs wrong result because always first one gets the worst result
# I just followed original one
# https://gist.github.com/timraymond/8d7014e0c7804f0fe508
memprof("Kernel.#Array") { CanCan::Rule.new(nil, [:read, :write], [:foo, :bar], nil, nil, __flag__: :array) }
memprof("Array#flatten") { CanCan::Rule.new(nil, [:read, :write], [:foo, :bar], nil, nil, __flag__: :flatten) }
memprof("Array.wrap")    { CanCan::Rule.new(nil, [:read, :write], [:foo, :bar], nil, nil, __flag__: :wrap) }

Benchmark.ips do |b|
  b.report("Kernel.#Array") { CanCan::Rule.new(nil, [:read, :write], [:foo, :bar], nil, nil, __flag__: :array) }
  b.report("Array#flatten") { CanCan::Rule.new(nil, [:read, :write], [:foo, :bar], nil, nil, __flag__: :flatten) }
  b.report("Array.wrap")    { CanCan::Rule.new(nil, [:read, :write], [:foo, :bar], nil, nil, __flag__: :wrap) }

  b.compare!
end

__END__
Ruby 2.6.1
[Kernel.#Array] Mem diff: 21.59765625 MB
[Array#flatten] Mem diff: 2.21484375 MB
[Array.wrap] Mem diff: 0.02734375 MB
Warming up --------------------------------------
       Kernel.#Array     5.024k i/100ms
       Array#flatten     3.863k i/100ms
          Array.wrap     5.217k i/100ms
Calculating -------------------------------------
       Kernel.#Array    204.954k (±21.6%) i/s -    964.608k in   5.003037s
       Array#flatten    143.892k (±17.3%) i/s -    695.340k in   5.005342s
          Array.wrap    195.569k (±18.5%) i/s -    933.843k in   5.003945s

Comparison:
       Kernel.#Array:   204954.3 i/s
          Array.wrap:   195569.0 i/s - same-ish: difference falls within error
       Array#flatten:   143892.4 i/s - same-ish: difference falls within error

Ruby 2.5.3
[Kernel.#Array] Mem diff: 21.98046875 MB
[Array#flatten] Mem diff: 0.93359375 MB
[Array.wrap] Mem diff: 0.0 MB
Warming up --------------------------------------
       Kernel.#Array     4.816k i/100ms
       Array#flatten     4.235k i/100ms
          Array.wrap     4.610k i/100ms
Calculating -------------------------------------
       Kernel.#Array    166.440k (±30.7%) i/s -    722.400k in   5.010362s
       Array#flatten    136.055k (±23.3%) i/s -    639.485k in   5.009729s
          Array.wrap    173.409k (±18.6%) i/s -    820.580k in   5.019846s

Comparison:
          Array.wrap:   173408.7 i/s
       Kernel.#Array:   166439.5 i/s - same-ish: difference falls within error
       Array#flatten:   136054.8 i/s - same-ish: difference falls within error

Ruby 2.4.5
[Kernel.#Array] Mem diff: 17.0859375 MB
[Array#flatten] Mem diff: 0.0234375 MB
[Array.wrap] Mem diff: 0.0 MB
Warming up --------------------------------------
      Kernel.#Array     4.746k i/100ms
      Array#flatten     4.019k i/100ms
          Array.wrap     4.347k i/100ms
Calculating -------------------------------------
      Kernel.#Array    195.099k (±17.4%) i/s -    939.708k in   4.994105s
      Array#flatten    117.881k (±19.3%) i/s -    562.660k in   5.012299s
          Array.wrap    160.679k (±13.9%) i/s -    782.460k in   5.005515s

Comparison:
      Kernel.#Array:   195099.4 i/s
         Array.wrap:   160678.6 i/s - same-ish: difference falls within error
      Array#flatten:   117881.1 i/s - 1.66x  slower
```

lib/cancan/rule.rb

@@ -19,8 +19,8 @@ def initialize(base_behavior, action, subject, conditions, block)
 
       @match_all = action.nil? && subject.nil?
       @base_behavior = base_behavior
-      @actions = Array(action)
-      @subjects = Array(subject)
+      @actions = wrap(action)
+      @subjects = wrap(subject)
       @conditions = conditions || {}
       @block = block
     end
@@ -93,5 +93,15 @@ def matches_subject_class?(subject)
           (subject.is_a?(Module) && subject.ancestors.include?(sub)))
       end
     end
+
+    def wrap(object)
+      if object.nil?
+        []
+      elsif object.respond_to?(:to_ary)
+        object.to_ary || [object]
+      else
+        [object]
+      end
+    end
   end
 end

spec/cancan/ability_spec.rb

@@ -424,6 +424,31 @@ class Container < Hash
     expect(@ability.attributes_for(:new, Range)).to eq(foo: 'foo', bar: 123, baz: 'baz')
   end
 
+  it 'allows to check ability even the object implements `#to_a`' do
+    class X
+      def self.to_a
+        []
+      end
+    end
+
+    @ability.can :read, X
+    expect(@ability.can?(:read, X.new)).to be(true)
+  end
+
+  it 'respects `#to_ary`' do
+    class X
+      def self.to_ary
+        [Y]
+      end
+    end
+
+    class Y; end
+
+    @ability.can :read, X
+    expect(@ability.can?(:read, X.new)).to be(false)
+    expect(@ability.can?(:read, Y.new)).to be(true)
+  end
+
   describe '#authorize!' do
     describe 'when ability is not authorized to perform an action' do
       it 'raises access denied exception' do