`clientMux`: Remove the implicit `Origin` check.

`websocket.Handler` implicitly checks the `Origin` header, causing
clients that do not send one (e.g., the Python WebSocket client) to
fail.
Add `ApplicationConfig.ResolveHost`.

This option is useful when running the server in a Docker container,
as other containers cannot resolve the server's hostname.
`sniffer()`: Normalize `RequestHeader.Type` case.