Avoid duping the String class when deep-duping

[rnubel]

On Rubinius, deep_dup'ing any structure containing the String class will explode with a TypeError; e.g.,

irb(main):004:0> { :foo => String }.deep_dup
TypeError: Unable to clone/dup String class
        from kernel/common/string.rb:39:in `dup (clone)'
        from /home/robert/Sandbox/ruby/rails/activesupport/lib/active_support/core_ext/object/deep_dup.rb:14:in `deep_dup'
        from /home/robert/Sandbox/ruby/rails/activesupport/lib/active_support/core_ext/object/deep_dup.rb:44:in `deep_dup'
        from kernel/common/enumerable.rb:83:in `each_with_object'
        from kernel/common/hash.rb:342:in `each'
        from kernel/common/enumerable.rb:81:in `each_with_object'
        from /home/robert/Sandbox/ruby/rails/activesupport/lib/active_support/core_ext/object/deep_dup.rb:42:in `deep_dup'
        from (irb):4

The cause of this is that String.dup cannot work in Rubinius, due to this code. Per a discussion with Rubinius' maintainer, that behavior is unlikely to change.

This patch fixes the issue by simply not dup-ing the String class (which could be made Rubinius-specific, if desired). To be honest, I'd rather we re-visit #20008, since dup-ing classes seems pretty pointless and produces unexpected results:

irb(main):004:0> NewString = String.dup
=> NewString
irb(main):005:0> v = NewString.new("test")
=> "test"
irb(main):006:0> v.is_a?(String)
=> false

Anyway, I've proposed this same change just for Grape in intridea/grape#1038, but it seemed like the type of thing to push upstream. Feedback welcome!

[arthurnn]

I wonder why that code is there in the first place (in Rubinius). I understand that dupping a base class like String could cause problems, but TBH, there are LOTS of other things can cause problems in ruby environment.
reading @brixen 's comment , seems like he is not sure why this was added to Rubinius too.

My opinion on this, is that, I dont wanna add a code into Rails that we are not sure WHY was there (in Rubinius), and we would not be sure why it is in Rails itself.

Thanks, though, for the PR, and good job investigating the problem