Promote SendRequest::pending to an OpaqueStreamRef. (#281)

Because `self.pending` doesn't necessarily get cleaned up in a timely fashion -
rather, only when the user calls `poll_ready()` - it was possible for it to
refer to a stream that has already been closed. This would lead to a panic the
next time that `poll_ready()` was called.

Instead, use an `OpaqueStreamRef`, bumping the refcount.

A change to an existing test is included which demonstrates the issue.

tests/h2-tests/tests/client_request.rs

@@ -916,6 +916,8 @@ fn notify_on_send_capacity() {
                     assert_eq!(response.status(), StatusCode::OK);
                 }
 
+                poll_fn(|| client.poll_ready()).wait().unwrap();
+
                 done_tx.send(()).unwrap();
             });