websocket: reset upload_done when sending data

Sending websocket data did not clear the "upload_done" flag of
the initial HTTP Upgrade request, leading to KEEP_SEND never be
cleared. This caused the socket to be polled for INOUT after all
the websocket data had been sent. A busy loop.

Closes #18476

lib/ws.c

@@ -1200,6 +1200,7 @@ CURLcode Curl_ws_accept(struct Curl_easy *data,
 
       /* start over with sending */
       data->req.eos_read = FALSE;
+      data->req.upload_done = FALSE;
       k->keepon |= KEEP_SEND;
     }