Disconnect on server-sent masked frames

[sanmiguel]

I don't have any stats on whether this really happens in the real world, but I've realised that the code handling unmasking of frames adds complexity and the RFC actually states (in Section 5.1) that:

a client MUST mask all frames that it sends to the server

and

A server MUST NOT mask any frames that it sends to the client. A client MUST close a connection if it detects a masked frame.

It would be quite pleasant to simply delete the complexity and enforce that server-sent frames MUST NOT be masked, and disconnect as per the RFC.