Convenience function to set the response body of a ServerMessage.
If content_type is null, the response body must be empty as well.
| this |
the message |
| content_type |
MIME Content-Type of the body |
| resp_use |
a MemoryUse describing how to handle |
| resp_body |
a data buffer containing the body of the message response. |
| resp_length |
the byte length of |