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

lib/transfer.c

@@ -2085,6 +2085,9 @@ CURLcode Curl_retry_request(struct connectdata *conn,
                                 prevent i.e HTTP transfers to return
                                 error just because nothing has been
                                 transfered! */
+
+    if(data->state.proto.http->writebytecount)
+      Curl_readrewind(conn);
   }
   return CURLE_OK;
 }