Prevent pending_open streams from being released. (#295)

* Prevent `pending_open` streams from being released.

This fixes a panic that would otherwise occur in some cases. A test
demonstrating said panic is included.

* Clear the pending_open queue together with everything else.

src/proto/streams/stream.rs

@@ -229,7 +229,7 @@ impl Stream {
             // The stream is not in any queue
             !self.is_pending_send && !self.is_pending_send_capacity &&
             !self.is_pending_accept && !self.is_pending_window_update &&
-            !self.reset_at.is_some()
+            !self.is_pending_open && !self.reset_at.is_some()
     }
 
     /// Returns true when the consumer of the stream has dropped all handles