Minor performance improvement; check first to see if it's a special key at all before the series of hasPrefix calls, since most of the time it will not be a special key.  The CFStringHasPrefix function did show up as a minor hotspot, and this helps a tiny bit.
Whoops, fix things for the "self" key.  And check for empty keys, though those are illegal with KV on regular objects -- could be OK for dictionaries.
Add comments per PR request
Merge pull request #2069 from carllindberg/2065-FasterSourceValueForKey

Speed up RKMappingSourceObject KeyValue