fixed bug #33962 TF_FIN not always set after FIN is sent. (This merely prevents nagle from not transmitting fast after closing.)

src/core/tcp_out.c

@@ -136,6 +136,7 @@ tcp_send_fin(struct tcp_pcb *pcb)
     if ((TCPH_FLAGS(last_unsent->tcphdr) & (TCP_SYN | TCP_FIN | TCP_RST)) == 0) {
       /* no SYN/FIN/RST flag in the header, we can add the FIN flag */
       TCPH_SET_FLAG(last_unsent->tcphdr, TCP_FIN);
+      pcb->flags |= TF_FIN;
       return ERR_OK;
     }
   }