Need to expose request.form() function

[mikefarah]

Request has a .form() function that can be called for multipart form data, documented:

https://github.com/request/request#forms

There are other ways to submit mulitpart forms (e.g. using the formData parameter) but as this is intended as a drop-in replace for request, it should expose that function too

[FGRibreau]

I will accept a PR for it along with updated tests.