Update 2xx http response success

See [rfc2616.10](http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html)
> ### 10.2 Successful 2xx
>
> This class of status code indicates that the client's request was successfully received, understood, and accepted.

An exception is raised when creating a group (HTTP status `201`) and when deleting members from a group (HTTP status `204`).  These should be considered successful responses.