Speed up RKDictionaryByMergingDictionaryWithDictionary a bit.  Primarily by avoiding an extra copy of the dictionary at the end, but also don't use -enumerate (has a nested autorelease pool), and use NSDictionary methods and not KV.
Revert change to RKObjectMapping metadata property.  This loses some of the performance gain (as each sub-operation will now make a copy) but it's less prounounced on a device.
Merge pull request #2068 from carllindberg/2065-SpeedUpDictionaryMerge

Speed up RKDictionaryByMergingDictionaryWithDictionary a bit.  Primarily...