Accept readable streams as "body"

[bendrucker]

The examples imply this functionality exists:

https://github.com/feross/simple-get#form-data

FormData returns a Readable, not a Buffer. The form data example will cause Node to throw with a "first argument must be a buffer or string" error when req.end is called with the stream.

I needed simple-get with support for streaming bodies at some point and ended up writing/publishing this:

https://github.com/bendrucker/stream-post

For supporting both streams and strings/buffers through the same interface accepting {body: readable} is the smallest working change.

[feross]

Perfect PR, thanks! Released as 2.6.0. I also added you as a collaborator to this repo since your PR was great.