Convenience function to set the request body of a Message.
If content_type is null, the request body must be empty as well.
| this |
the message |
| content_type |
MIME Content-Type of the body |
| req_use |
a MemoryUse describing how to handle |
| req_body |
a data buffer containing the body of the message request. |
| req_length |
the byte length of |