Expose IndifferentAccess conversion internally

In some cases --- like writing meta-extensions of `IndifferentAccess`
--- we need access to the ability to convert a key for the purposes of
`IndifferentAccess`. Exposing this behavior as a module function makes
this possible.

lib/hashie/extensions/indifferent_access.rb

@@ -25,6 +25,11 @@ module Extensions
     module IndifferentAccess
       include Hashie::Extensions::RubyVersionCheck
 
+      # @api private
+      def self.convert_key(key)
+        key.to_s
+      end
+
       def self.included(base)
         Hashie::Extensions::Dash::IndifferentAccess.maybe_extend(base)
 
@@ -68,7 +73,7 @@ def self.inject(hash)
       end
 
       def convert_key(key)
-        key.to_s
+        IndifferentAccess.convert_key(key)
       end
 
       # Iterates through the keys and values, reconverting them to