feat(client,server) Add Connection::without_shutdown()

* Add `server::conn::Connection::without_shutdown`

  Returns wrapper Future instance which allows
  to use `poll_without_shutdown` method
  more ergonomically.

* Add `client::conn::Connection::without_shutdown`

  Returns wrapper Future instance which allows
  to use `poll_without_shutdown` method
  more ergonomically.

* Improve `poll_without_shutdown` docs

Closes #1786

src/client/conn.rs

@@ -376,6 +376,10 @@ where
     /// upgrade. Once the upgrade is completed, the connection would be "done",
     /// but it is not desired to actally shutdown the IO object. Instead you
     /// would take it back using `into_parts`.
+    ///
+    /// Use [`poll_fn`](https://docs.rs/futures/0.1.25/futures/future/fn.poll_fn.html)
+    /// and [`try_ready!`](https://docs.rs/futures/0.1.25/futures/macro.try_ready.html)
+    /// to work with this function; or use the `without_shutdown` wrapper.
     pub fn poll_without_shutdown(&mut self) -> Poll<(), ::Error> {
         match self.inner.as_mut().expect("already upgraded") {
             &mut Either::A(ref mut h1) => {
@@ -386,6 +390,16 @@ where
             }
         }
     }
+
+    /// Prevent shutdown of the underlying IO object at the end of service the request,
+    /// instead run `into_parts`. This is a convenience wrapper over `poll_without_shutdown`.
+    pub fn without_shutdown(self) -> impl Future<Item=Parts<T>, Error=::Error> {
+        let mut conn = Some(self);
+        ::futures::future::poll_fn(move || -> ::Result<_> {
+            try_ready!(conn.as_mut().unwrap().poll_without_shutdown());
+            Ok(conn.take().unwrap().into_parts().into())
+        })
+    }
 }
 
 impl<T, B> Future for Connection<T, B>