Add RecvStream::is_end_stream. (#165)

This function returns true if the `RecvStream` has reached an end of
stream state. This is intended to replace `is_empty` which has confusing
behavior.

tests/flow_control.rs

@@ -161,6 +161,7 @@ fn release_capacity_of_small_amount_does_not_send_window_update() {
                 .and_then(|resp| {
                     assert_eq!(resp.status(), StatusCode::OK);
                     let body = resp.into_parts().1;
+                    assert!(!body.is_end_stream());
                     body.into_future().unwrap()
                 })
                 // read the small body and then release it

tests/stream_states.rs

@@ -169,7 +169,9 @@ fn closed_streams_are_released() {
             // stream wired.
             assert_eq!(1, client.num_wired_streams());
 
-            drop(response);
+            let (_, body) = response.into_parts();
+            assert!(body.is_end_stream());
+            drop(body);
 
             // The stream state is now free
             assert_eq!(0, client.num_wired_streams());