Merge pull request #100 from Trilliant/Trilliant/feature/fixLwipRelativeTimeOnExpiration

Set relative time 't' to zero when it actually expire

src/net.c

@@ -2019,6 +2019,7 @@ static void coap_retransmittimer_execute(void *arg) {
       break;
     } else {
       elapsed -= nextinqueue->t;
+      nextinqueue->t = 0;
       coap_retransmit(ctx, coap_pop_next(ctx));
       nextinqueue = coap_peek_next(ctx);
     }