message: chore(client): fix unused_mut in 1.18
diff: @@ -226,7 +226,7 @@ impl Future for HttpConnecting {
  };
  },
  State::Connecting(ref mut c) => {
  - let mut sock = try_ready!(c.poll(&self.handle));
  + let sock = try_ready!(c.poll(&self.handle));
  if let Some(dur) = self.keep_alive_timeout {
  sock.set_keepalive(Some(dur))?;
  