Hash with indifferent access reverse merge problem

My business partner Rodney ran into this isse the other day.

http://blog.internautdesign.com/...

I think it is a bug, that violates the Principle of Least Surprise.

Loading development environment (Rails 2.1.0)
>> z = HashWithIndifferentAccess.new
=> {}
>> z.reverse_merge!( :a => 1 )
=> {:a=>1}
>> z.reverse_merge!( 'a' => 1 )
=> {"a"=>1, :a=>1}

Reverse merging on a HashWithIndifferentAccess should return a new HashWithIndifferentAccess, not a normal hash.

The solution is to overload HashWithIndifferentAccess#reverse_merge:

>> class HashWithIndifferentAccess
>>   def reverse_merge(other_hash)
>>     self.class.new( other_hash.merge(self) )
>>   end
>> end
=> nil
>> z = HashWithIndifferentAccess.new
=> {}
>> z.reverse_merge!( :a => 1 )
=> {"a"=>1}
>> z.reverse_merge!( 'a' => 1 )
=> {"a"=>1}
>> 

I'm attaching a patch with a test.