Don't try to dup the String class.

Rubinius will raise a TypeError, should you try String.dup, whereas
MRI has no problem with it. As such, this change marks String as un-
duplicable with respect to ActiveSupports' #duplicable? method, used
by #deep_dup. Since this change isn't in upstream ActiveSupport, I
also forced the backport to always be loaded (this could be changed
to just a monkeypatch of the String class, though).

Author: rnubel

CHANGELOG.md

@@ -1,8 +1,14 @@
 Next Release
 ============
 
+#### Features
+
 * Your contribution here.
 
+#### Fixes
+
+* [#1038](https://github.com/intridea/grape/pull/1038): Avoid dup-ing the String class when used in inherited params - [@rnubel](https://github.com/rnubel).
+
 0.12.0 (6/18/2015)
 ==================
 

lib/backports/active_support/duplicable.rb

@@ -73,6 +73,15 @@ def duplicable?
     false
   end
 end
+class String
+  class << self
+    # Rubinius explicitly disallows this. Also, there's no real
+    # reason you'd want to duplicate the String class in a deep_dup.
+    def duplicable?
+      false
+    end
+  end
+end
 require 'bigdecimal'
 # rubocop:disable Lint/HandleExceptions
 class BigDecimal

lib/grape.rb

@@ -15,6 +15,7 @@
 else
   require_relative 'backports/active_support/deep_dup'
 end
+require_relative 'backports/active_support/duplicable'
 
 require 'active_support/ordered_hash'
 require 'active_support/core_ext/object/conversions'

spec/grape/util/inheritable_values_spec.rb

@@ -58,6 +58,13 @@ module Util
           expect(subject.to_hash).to eq(some_thing: :foo, some_thing_more: :foo_bar)
         end
       end
+
+      describe '#clone' do
+        it 'clones itself even when containing a String class' do
+          subject[:foo] = String
+          expect(subject.clone.to_hash).to eq(foo: String)
+        end
+      end
     end
   end
 end