SecureChannels.margin is a Generic Subscription, doesn't work with Symbol

[throughnothing]

When using SecureChannels.margin I noticed that I didn't seem to be getting the margin feed.

Looking at the Bitmex WebSocket client here: https://github.com/BitMEX/api-connectors/blob/master/official-ws/python/bitmex_websocket.py#L164

I noticed that they don't add the symbol to margin, and call it a "generic" sub. I think this needs to be handled properly here to allow margin to be subscribed to successfully.

This code: https://github.com/joliveros/bitmex-websocket/blob/master/bitmex_websocket/_instrument.py#L42

Likely needs to know if a channel is "generic" or not, to know whether or not to add the :SYMBOL.