Prefer the use RKAttributeMapping value transformers if provided instead of defaulting to ones from RKEntityMapping

When using custom RKValueTransformers with RKEntityMapping they are ignored during mapping. With transformed identification attributes this leads to duplicate records.
Changed mapping operation to use transformers provided by attribute mapping, if any.
Unbraced conditional in 1 line
Added test for custom value transformer on identification attribute
Merge pull request #1820 from FWKamil/development

Custom value transformers on identification attributes are ignored during mapping