fixes #2000 string identifier

The problem was more in depth!

Basically when the `sourceKeyPath` is `nil` the correct value get embedded into an `RKMappingSourceObject` but then the whole object is passed to `applyAttributeMapping:withValue:' which for some reasons does not crash, but it adds a completely irrelevant string composed by value (attribute metadata) to the destination object.

This makes any kind of comparison for a certain identifier failing.

The fix consist in passing the right value to `applyAttributeMapping:withValue:' when the source key path is `nil`.

The right value is stored inside the `RKMappingSourceObject` inside the object attribute.
Why is this the right selector?