Return Buffer if encoding is undefined

I just ran into issue #823 today as well. It seems like it makes the most sense to have not providing an `encoding` value (i.e. `encoding` is `undefined`) do the same thing as setting `encoding` to `null`. This can be easily done by changing [one character](https://github.com/mikeal/request/blob/master/request.js#L967) (`===` to `==`).