Skeleton code for the data merging routine to be used in edit actions
Added a few tests for Marshaller::merge()
Adding tests for merging one to many associations and fixing issue
related to it
Adding test for merging belongsToMany in existing entity when passing
_ids
Implemented merging of _joinData in the marshaller
Fixed merging associations inside _joinData
Added test for Marshaller::mergeMany
Added tests to show how mergeMany discards unmatched records
Documenting new methods and fixing a couple CS issues