RKAttributeMapping lose custom value transformer when RKObjectMapping is inverted

In RKObjectMapping
On invertMapping:withPredicate:

Each mapping.attributeMappings is recreated with source/destination keyPaths inverted but its propertyValueClass and valueTransformer are not copied from original attributeMapping

Solution:
Implemented and used a new copyPropertiesFromMapping: method for RKAttributeMapping in the same way it already was for RKObjectMapping
Actually, I'm not sure we should be copying `propertyValueClass`