encoding null returns Buffer on linux but string on windows!

[Zibri]

                    if (typeof process.env.ComSpec == "undefined") {
                        cmd = "bash";
                        cmdargs = ["-i"]
                    } else {
                        cmd = "cmd.exe";
                        cmdargs = []
                    }

mypty = pty.spawn( cmd, cmdargs, {
                    name: 'xterm-256color',
                    cols: 80,
                    rows: 25,
                    env: process.env,
                    encoding: null
                });

if cmd and cmdargs is bash -i, with encoding:null mypty.on("data") returns a Buffer as it should. but if running on windows and cmd and cmdargs are CMD.EXE then mypty.on("data") returns a STRING!

[Tyriar]

Right now we have this as a warning:

https://github.com/microsoft/node-pty/blob/3794fb111c51f2fa03ee6150468cc462db76699a/src/windowsTerminal.ts#L33-L35

Though it won't show up if you pass in null, we should fix that and also update the .d.ts to explain.