http/request: reset body mbuf pos on re-sending

[maximilianfridrich]

This fixes the following bug:

If a HTTP request is resent (e.g. with Authentication after a 401 response), the body of the request is not sent.

This is because in req_body_handler mbuf_advance(conn->body, len) is called. Then the request is re-sent and the body size is obtained by calling mbuf_get_left(conn->body) which is now incorrect.

The fix is to reset the mbuf position of conn->body before re-sending a request.