Changes to Create a return line item

added the new required request property data/relationships/line_item/data

added the new required request property data/relationships/return/data

request property data/type was restricted to a list of enum values

removed the request property data/relationships/line_item/id

removed the request property data/relationships/line_item/type

removed the request property data/relationships/return/id

removed the request property data/relationships/return/type

the type request property default value return_line_items was removed

added the new return_line_items enum value to the request property data/type

added the media type application/vnd.api+json for the response with the status 201