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/stream.rs

@@ -23,6 +23,7 @@ pub(super) struct Stream<B> {
     pub requested_send_capacity: WindowSize,
 
     /// Amount of data buffered at the prioritization layer.
+    /// TODO: Technically this could be greater than the window size...
     pub buffered_send_data: WindowSize,
 
     /// Task tracking additional send capacity (i.e. window updates).