message: docs(client): `send_request` doc test to async/await
diff: @@ -191,16 +191,13 @@ where
  /// # Example
  ///
  /// ```
  - /// # extern crate futures;
  - /// # extern crate hyper;
  - /// # extern crate http;
  + /// # #![feature(async_await)]
  /// # use http::header::HOST;
  /// # use hyper::client::conn::SendRequest;
  /// # use hyper::Body;
  - /// use futures::Future;
  /// use hyper::Request;
  ///
  - /// # fn doc(mut tx: SendRequest<Body>) {
  + /// # async fn doc(mut tx: SendRequest<Body>) -> hyper::Result<()> {
  /// // build a Request
  /// let req = Request::builder()
  /// .uri("/foo/bar")
  @@ -208,13 +205,11 @@ where
  /// .body(Body::empty())
  /// .unwrap();
  ///
  - /// // send it and get a future back
  - /// let fut = tx.send_request(req)
  - /// .map(|res| {
  - /// // got the Response
  + /// // send it and await a Response
  + /// let res = tx.send_request(req).await?;
  + /// // assert the Response
  /// assert!(res.status().is_success());
  - /// });
  - /// # drop(fut);
  + /// # Ok(())
  /// # }
  /// # fn main() {}
  /// ```
  