Pass the request body (don't assume this.request.body set)

This addresses the same issue as [pull request #8](https://github.com/popomore/koa-proxy/pull/8), but with a different approach.

A proxy should preserve the original request as much as possible. It's not safe to assume what body parser (if any) an application is using, and it's not ideal to re-stringify a body that's already been parsed to an object. Instead, it seems preferable to get the original, raw request body and pass it through to the destination server.

I took a slightly different approach to testing this as well. The key thing to validate is that the original request is passed-through to the destination server unchanged – so rather than validate a response from the server, I just validate the request body in the destination app's middleware. (This required creating a separate destination app than the one in the setup).

Let me know what you think.