cmd_send: wait for ack from close()

Without this, the sender drops the connection before the "close" message
has made it to the server, which leaves the mailbox hanging until it
expires. It still lives in a 'd.addBoth()' slot, so it gets closed even
if some error occurrs, but we wait for it's Deferred to fire in both
success and failure cases.

src/wormhole/cli/cmd_send.py

@@ -48,10 +48,9 @@ class Sender:
         w = wormhole(APPID, self._args.relay_url,
                      self._reactor, self._tor_manager,
                      timing=self._timing)
-        try:
-            yield self._go(w)
-        finally:
-            w.close()
+        d = self._go(w)
+        d.addBoth(w.close) # must wait for ack from close()
+        yield d
 
     def _send_data(self, data, w):
         data_bytes = dict_to_bytes(data)