Changes to Create a coupon recipient

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

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

removed the request property data/relationships/customer/id

removed the request property data/relationships/customer/type

the type request property default value coupon_recipients was removed

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

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