Added PrepareWebSocketClient option

[aspnet-hello]

From @saithis on Tuesday, November 28, 2017 12:23:10 AM

There was already a PrepareRequest for HttpRequests and with this we have the same for websockets. This PR replaces #75 as the rest of #75 is already possible in a better way, as described by @mkosieradzki .

Copied from original issue: aspnet/Proxy#76

[saithis]

Why was this closed? (@aspnet-hello ?)

[aspnet-hello]

This was an accidental issue created from this PR: aspnet/Proxy#76

So this issue isn't needed - just use the original PR at aspnet/Proxy#76.