ManagedWebSocket doesn't send payloads if `_isServer` is true

[analogrelay]

Looks like System.Net.WebSockets.ManagedWebSocket.WriteFrameToSendBuffer fails to actually write the payload if _isServer is set. It copies the payload into the send buffer in the same if-branch that handles masking (which is client-only 😄).

Discovered and fixed this while porting code to ASP.NET, so I'm going to port the fix over to corefx and open a PR.

/cc @stephentoub @Tratcher

[analogrelay]

Also, the Mask is not properly consumed from the receive buffer so received messages end up with 4 \0 characters at the start. I have a fix for this as well. Just struggling to find where and how to add tests to the corefx code. I'm making progress though 😄

[joshfree]

@davidsh @CIPop

[analogrelay]

FYI I have a PR with the fix: dotnet/corefx#10510