set_request_body
Description:
Set the request body of a Message.
If content_type is null and stream is not null 
      the Content-Type header will not be changed if present. The request body needs to be set again in case this is
       restarted (in case of redirection or authentication).
Parameters:
| this | the message | 
| content_type | MIME Content-Type of the body, or null if unknown | 
| stream | a InputStream to read the request body from | 
| content_length | the byte length of  |