Call abs on arity when extracting reload

[bradylove]

Fixes #23

• Optional arguments in the method signature cause arity to be a
  negative number. Call abs on the method arity to fix optional values not
  working and throwing something like:

  ArgumentError: wrong number of arguments (3 for 2)

[gamov]

👍

[dark-panda]

This appears to break some method prototypes. For instance...

class Foo
  extend Memoist

  def two_optional_args(i = 0, j = false)
    "#{i} #{j}"
  end
  memoize :two_optional_args
end

Foo.new.two_optional_args("one", false)
# "one false"

Foo.new.two_optional_args("one", true) 
# "one false", but "one true" expected