Expose writeControl

We all had to deal with weird external websocket configs, well the one I have the pleasure to deal with this time requires me to send a PONG frame every X seconds (there's no ping) to keep the connection alive.

It would be handy to expose writeControl for this.

Perhaps for the next version?

Cheers :heart:

Locate the existing writeControl implementation and the public connection API in this Go WebSocket repository. Check how control frames are tested, then expose the requested operation and add coverage showing that callers can send a PONG frame; done means the API is publicly usable for periodic PONG messages.

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