Fix send flow control bug

The send stream state is transitioned before data is buffered. As such,
the stream state could be closed while there is still data to be sent.

src/proto/streams/prioritize.rs

@@ -159,10 +159,8 @@ impl<B> Prioritize<B>
                                      stream: &mut store::Ptr<B>)
         -> Result<(), ConnectionError>
     {
-        // Ignore window updates when the stream is not active.
-        if !stream.state.could_send_data() {
-            return Ok(());
-        }
+        trace!("recv_stream_window_update; stream={:?}; state={:?}; inc={}; flow={:?}",
+               stream.id, stream.state, inc, stream.send_flow);
 
         // Update the stream level flow control.
         stream.send_flow.inc_window(inc)?;
@@ -222,8 +220,9 @@ impl<B> Prioritize<B>
         }
 
         // If the stream has requested capacity, then it must be in the
-        // streaming state.
-        debug_assert!(stream.state.is_send_streaming());
+        // streaming state (more data could be sent) or there is buffered data
+        // waiting to be sent.
+        debug_assert!(stream.state.is_send_streaming() || stream.buffered_send_data > 0);
 
         // The amount of currently available capacity on the connection
         let conn_available = self.flow.available();