Differentiate on data types rather than fields

[bogdanadrianmarc]

This issue is related to this comment: epimorphics/ds-api-ruby#8 (comment) , and the original issue for adding JSON complete mode: epimorphics/ds-api-ruby#7. Currently for JSON complete mode, special cases such as refMonth are differentiated by their field names, this needs to be changed to differentiate based on data types, such as YearMonth, Date, etc. This way if another similar data type is introduced in the future, we won't be needing to add an extra special case for converting the response.

This is currently on hold waiting for JSON-LD support from SAPINT