Fix flow control bug (#177)

This patch fixes a bug that prevents sent data from being flushed to the
socket.

When data is sent, the task managing the connection must be notified. A
guard exists that prevents unnecessary notification of the connection
when the stream does not have any send capacity. However, this guard was
buggy. Instead of notifying the connection if *any* data can be sent, it
notified the connection only when *all* data could be sent.

This patch fixes the check as well as adds some tests that ensure the
connection task is notified.

tests/support/mod.rs

@@ -52,6 +52,7 @@ pub mod raw;
 pub mod frames;
 pub mod prelude;
 pub mod mock;
+pub mod notify;
 pub mod util;
 
 mod future_ext;