sending post_data and receiving Buffers in the content functionality

While fully aware that it's an old topic, i don't see why oauth should operate on utf-8 data, since encoding should be a matter discussed between client application and server.
And since oauth is a kind of middleware, the buffers should be exactly what's needed, since they pass the data as is. Byte by byte.
If the client want's data sent or received in utf8, there's no reason why not to prepare utf data, and send new Buffer(data, 'utf8').

I can't think of a fail scenario right now, but in case you're concerned for old applications i prepared another pull request for that, feel free to choose.