Curl_pgrsStartNow: keep HEADERS_OUT set

To avoid that the progress meter headers get output between each
transfer, make sure the bits gets kept when (re-)inited.

Reported by: Christopher Stone

lib/progress.c

@@ -201,7 +201,8 @@ void Curl_pgrsStartNow(struct SessionHandle *data)
 {
   data->progress.speeder_c = 0; /* reset the progress meter display */
   data->progress.start = Curl_tvnow();
-  data->progress.flags &= PGRS_HIDE; /* clear all bits except HIDE */
+  /* clear all bits except HIDE and HEADERS_OUT */
+  data->progress.flags &= PGRS_HIDE|PGRS_HEADERS_OUT;
 }
 
 void Curl_pgrsSetDownloadCounter(struct SessionHandle *data, curl_off_t size)