Avoid returning 0, nil in netConn.Read

[MarcoPolo]

When we reach the end of the frame we currently return 0, nil (https://github.com/nhooyr/websocket/blob/master/netconn.go#L118). The io.Reader docs discourage this:

Implementations of Read are discouraged from returning a zero byte count with a nil error, except when len(p) == 0.

This seems to have confused other folks as well: #353

My recommendation is we don't return 0, nil and instead block until we get data or a proper error.

[nhooyr]

Will change because the docs for io.Reader discourage it but if your code is using io.Reader correctly this should never be a problem.

[MarcoPolo]

How do you think you want to solve it? Loop until error or data? Or something else?

I'm happy to submit a patch.

[nhooyr]

Related golang/go#27531 (comment)

[nhooyr]

Fixed in dev.