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/settings.rs

@@ -36,6 +36,8 @@ impl Settings {
               B: Buf,
               C: Buf,
     {
+        trace!("send_pending_ack; pending={:?}", self.pending);
+
         if let Some(ref settings) = self.pending {
             let frame = frame::Settings::ack();
 
@@ -44,7 +46,7 @@ impl Settings {
                 return Ok(Async::NotReady);
             }
 
-            trace!("ACK sent");
+            trace!("ACK sent; applying settings");
 
             dst.apply_remote_settings(settings);
             streams.apply_remote_settings(settings)?;