message: refactor(http): grab io.write_buf_mut only once in write_head
diff: @@ -248,14 +248,15 @@ where I: AsyncRead + AsyncWrite,
  let wants_keep_alive = head.should_keep_alive();
  self.state.keep_alive &= wants_keep_alive;
  + let mut buf = self.io.write_buf_mut();
  // if a 100-continue has started but not finished sending, tack the
  // remainder on to the start of the buffer.
  if let Writing::Continue(ref pending) = self.state.writing {
  if pending.has_started() {
  - self.io.write_buf_mut().extend_from_slice(pending.buf());
  + buf.extend_from_slice(pending.buf());
  }
  }
  - let encoder = T::encode(head, self.io.write_buf_mut());
  + let encoder = T::encode(head, buf);
  self.state.writing = if body {
  Writing::Body(encoder, None)
  } else {
  