Support data field as body for link operation

Signed-off-by: Clark Wang <clark.wangs@gmail.com>
Add tests for hasManyThrough link with data

Signed-off-by: Clark Wang <clark.wangs@gmail.com>
Restrict that only hasManyThrough can have additional properties

Signed-off-by: Clark Wang <clark.wangs@gmail.com>
Restrict: only hasManyThrough relation can have additional properties

Signed-off-by: Clark Wang <clark.wangs@gmail.com>
Merge pull request #522 from clarkorz/feature/link-with-data

Support data field as body for link operation