[RFC] convenience function to check if websocket has incoming messages

[cpfiffer]

I need to be able to check if a websocket connection has an incoming message, and I want to open a PR to HTTP.jl to do so. I mostly just want to know the method name you'd suggest, though if there is a more idiomatic/correct way to check whether a message is available I am all ears.

I know that you can access the incoming bytes of a WebSocket with

has_message = !isempty(ws.readbuffer)

Some candidates for this function name:

• isempty(ws::WebSocket)
• eof(ws::WebSocket) would be semi-idiomatic here, but the IOBuffer has already been read into readbuffer (I think?)

Perhaps we don't need a convenience function here, but I'd at least like to add something to the documentation string.