Adding ability to send node.js Buffer

As far as I could tell, sending a raw binary `Buffer` is not currently doable in needle, but it can be supported with very little modification.
It's not really clear. According to [the documentation](http://nodejs.org/api/http.html#http_response_write_chunk_encoding), "`chunk` can be a string or a buffer. If `chunk` is a string, the second parameter specifies how to encode it into a byte stream.". It makes no comment about the `encoding` parameter when `chunk` is a `Buffer`.

The [source code](https://github.com/joyent/node/blob/master/lib/_http_outgoing.js) also doesn't make it particularly clear. I think it falls back on the `"binary"` encoding (which is deprecated according to the node.js documentation).

In any case, with the change that I made, I was able to use needle to send my binary payload correctly.