MockWebServer: Support closing WebSocket with codes 1005 or 1006

[gnarea]

I'm using MockWebServer to test a WebSocket client which should be able to gracefully deal with an scenario where the server closes the connection without a status code (1005) or doesn't send a close frame at all (1006).

I'd expect to be able to call websocket.close() to replicate the former, but WebSocket.close() requires a code. But if I set 1005, OkHTTP rejects it (as it should per the spec). That means I don't have a way to replicate a 1005.

On the other hand, I'd expect to be able to replicate a 1006 by calling websocket.cancel(). However, I get a NullPointerException because RealWebSocket.cancel() seems to be reserved for client sockets:

okhttp/okhttp/src/main/kotlin/okhttp3/internal/ws/RealWebSocket.kt

Lines 70 to 71 in 9ee3344

	/** Non-null for client web sockets. These can be canceled. */
	private var call: Call? = null

okhttp/okhttp/src/main/kotlin/okhttp3/internal/ws/RealWebSocket.kt

Lines 142 to 144 in 9ee3344

	override fun cancel() {
	call!!.cancel()
	}