Changes to Create a coupon

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

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

removed the request property data/relationships/promotion_rule/id

removed the request property data/relationships/promotion_rule/type

added the new optional request property data/attributes/expires_at

added the new optional request property data/relationships/coupon_recipient

added the new optional request property data/relationships/tags

the request property data/attributes/usage_limit became optional

the type request property default value coupons was removed

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

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