Making a few tweaks to RKMappingOperation applyRelationshipMappings.  First, there is a temporary array created which ends up only being used for its count, so changed to an NSUInteger.  Just a small speedup, but it's there.  In another section, the code was calling valueForKeyPath "mapping.propertyMappings.sourceKeyPath" on an RKRelationshipMapping, which if it happens to be an RKDynamicMapping, I think would fail.  Code immediately above has already determined the RKObjectMapping, so it's easier to just call the method directly, and iterate using the RKPropertyMappings directly, which avoids another temporary array allocation.  Also avoided a double call to RKObjectIsCollection though that is a minimal speedup.
Merge pull request #2082 from carllindberg/2065-ApplyRelationshipMappings

Few changes to applyRelationshipMappings