Handle nil value transformers
Assert against nil transformers
Document non-nil requirement
WS
Make transformer for key consistent with convenience methods
Add MTLIdentityValueTransformerName
Revert "Add MTLIdentityValueTransformerName"

This reverts commit 8e80404b3dda92533991104737e31e1c544da80d.
Revert "Make transformer for key consistent with convenience methods"

This reverts commit 598d5caf96b9dc9de231fcc9b607f1c13fa11281.
Revert "WS"

This reverts commit 69ac21101a1243a635e8a1f7a855297b6df095f1.
Revert "Document non-nil requirement"

This reverts commit f3f62de1cb4a49bf0af76c1bb56fac96cbde4cb3.
Revert "Assert against nil transformers"

This reverts commit 1ca75a9d6c8b1816cc69146751b18b408d5b529e.
Handle nil from JSONTransformerForKey:

and entityAttributeTransformerForKey:, too
Merge pull request #320 from Mantle/nil-transformers

Handle nil transformers