Very minor typo fix
Merge pull request #211 from ColinEberhardt/master

Very minor typo fix
Return value for [NSNull null] if key is null in mtl_valueMappingTransformer
Merge pull request #212 from StyleShare/master

Fix a crash when key is null in mtl_valueMappingTransformer
Merge branch 'master' into 2.0-development

Conflicts:
	Mantle/NSValueTransformer+MTLPredefinedTransformerAdditions.m