retry-request: rewind if data was sent

When libcurl sends a HTTP request on a re-used connection and detects it
being closed (ie no data at all was read from it), it is important to
rewind if any data in the request was sent using the read callback or
was read from file, as otherwise the retried request will be broken.

Reported by: Chris Smowton
Bug: http://curl.haxx.se/bug/view.cgi?id=3195205