Websocket Subscription support

Hi!
I'm testing the library. It has websocket support, but when I tried to subscribe to a blockchain node through a `subscribe...` method, it didn't worked. When debbuging, I noticed the received message was going to the `default` case in the `handleFrame` func ([ref](https://github.com/filecoin-project/go-jsonrpc/blob/bbbae8641e516b378dacb95531b41e7fdef4ff4c/websocket.go#L527)). I believe one of the problems is that the switch only consider the message as a respone if the `method` field is empty, but in a subscription response the method usually is `domain_subscriptionMethodName`.

So, the library doens't support subscription, am I correct?
If so, is there any plans to implement it?