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/prelude.rs

@@ -13,6 +13,9 @@ pub use super::mock::{self, HandleFutureExt};
 // Re-export frames helpers
 pub use super::frames;
 
+// Re-export mock notify
+pub use super::notify::MockNotify;
+
 // Re-export utility mod
 pub use super::util;