Hijack connection through http.ResponseController in http upgrader

[xakep666]

Go 1.20 introduced new and more convenient way to control response and discover features - ResponseController. It's methods can introspect middlewares implementing following interface

type rwUnwrapper interface {
	Unwrap() ResponseWriter
}

source

This PR contains changes to use it if go version newer than 1.20. Old behaviour will be used if go version is older than 1.20.

[cristaloleg]

Nice one. Can you drop the newline before return in both hijack functions please?

[xakep666]

@cristaloleg done

[cristaloleg]

Thanks @xakep666 I will tag 1.3.0 a bit later today.

[cristaloleg]

And done https://github.com/gobwas/ws/releases/tag/v1.3.0

[jvilhuber]

Note go 1.21 is out, so the test "!go1.20" for go 1.19 is no longer correct.

[xakep666]

@jvilhuber "go 1.20", means "go 1.20 or newer" so negated one is "go 1.19 or older"

[jvilhuber]

Thanks! Learned something new today!