Close frames echoed back to the client masked
In the current master branch, if the client sends a close frame to the server, the server will echo back the frame with the mask retained. This breaks the protocol specification and causes some clients to complain. In this particular instance, [SocketRocket](https://github.com/square/SocketRocket) was complaining.

The new 0.4 branch does not exhibit this behavior.

This was found while debugging: https://github.com/voryx/Thruway/issues/194
