allow using a wrapped net.TCPConn in NewConn

[marten-seemann]

Currently, the connection passed to NewConn has to be a *net.TCPConn, even though the function allows for a net.Conn to be passed. This is because NewConn performs a type assertion:

tcp/rawconn.go

Lines 99 to 112 in 803a9b4

	func NewConn(c net.Conn) (*Conn, error) {
	cc := &Conn{Conn: c}
	switch c := c.(type) {
	case *net.TCPConn:
	var err error
	cc.c, err = c.SyscallConn()
	if err != nil {
	return nil, err
	}
	return cc, nil
	default:
	return nil, errors.New("unknown connection type")
	}
	}

It would be nice if it was possible to pass a wrapped *net.TCPConn to NewConn. This should be a problem, as the only function actually needed is SyscallConn(). By doing an interface assertion using an interface only fulfilled by a net.TCPConn, we can even make sure that we don't accidentally use this package with a connection that's not a TCP connection.

[marten-seemann]

@mikioh Any thought about this PR?

[marten-seemann]

@mikioh Friendly ping.
It would be really great if we could get this PR merged soon, I'd hate to fork this project for such a minor change.