SetBodyStreamWriter registers the given sw for populating request body.
This function may be used in the following cases:
* if request body is too big (more than 10MB). * if request body is streamed from slow external sources. * if request body must be streamed to the server in chunks (aka `http client push` or `chunked transfer-encoding`).
Note that GET and HEAD requests cannot have body.
/ See also SetBodyStream.