✅ Fix ruby 2.7 compatibility

ruby 2.7 didn't have:
* Hash#except.
* pattern matching statements: "foo => a | b | c"

lib/net/imap/config.rb

@@ -281,7 +281,9 @@ def with(**attrs)
       def load_defaults(version)
         [Numeric, Symbol, String].any? { _1 === version } or
           raise ArgumentError, "expected number or symbol, got %p" % [version]
-        update(**Config[version].to_h.except(*DEFAULT_TO_INHERIT))
+        config = Config[version]
+        defaults = config.to_h.reject {|k,v| DEFAULT_TO_INHERIT.include?(k) }
+        update(**defaults)
       end
 
       # :call-seq: to_h -> hash
@@ -300,7 +302,7 @@ def to_h; data.members.to_h { [_1, send(_1)] } end
       @global = default.new
 
       version_defaults[0.4] = Config[
-        default.to_h.except(*DEFAULT_TO_INHERIT)
+        default.to_h.reject {|k,v| DEFAULT_TO_INHERIT.include?(k) }
       ]
 
       version_defaults[0] = Config[0.4].dup.update(

test/net/imap/test_config.rb

@@ -137,7 +137,7 @@ class ConfigTest < Test::Unit::TestCase
 
   test ".version_defaults are all frozen, and inherit debug from global" do
     Config.version_defaults.each do |name, config|
-      name => 0 | Float | Symbol
+      assert [0, Float, Symbol].any? { _1 === name }
       assert_kind_of Config, config
       assert config.frozen?,            "#{name} isn't frozen"
       assert config.inherited?(:debug), "#{name} doesn't inherit debug"