ClientWebSocket: SendAsync endOfMessage inverted.

endOfMessage should indicate whether or not additional message parts will follow the current one. Currently, the meaning in ClientWebSocket.cs is inverted: !endOfMessage indicates that no more parts will follow the current one.

See the WebSockets RFC, 'frame-fin' at the top of http://tools.ietf.org/html/rfc6455#page-31.
Also see the MSDN docs for SendAsync: http://msdn.microsoft.com/en-us/library/system.net.websockets.websocket.sendasync(v=vs.110).aspx
Unfortunately we don't have this method in Mono yet, it was implemented in PR #981 but had to be reverted because it caused test failures.

I checked that the test works on .NET, so I think the best we can do for now is to disable the test until we have support for that method (apply ```[Category ("NotWorking")]``` to the test).