Default headers argument to stream.headers

[PaulKiddle]

Thanks for this neat little library!
One of the example usages for this is req => form(req, req.headers) - it strikes me that this probably common use case could be terser if we assume that the stream's headers are on stream.headers if the second argument is absent.

[blakeembrey]

I'd be fine accepting a PR. I don't use this package with node.js request streams so it's not helpful to me, I just made this generic enough that anyone using any node.js stream should be able to use it.