Feature request - Implement multipart request

[bunnywrote]

At the moment the method BaseRequest toHttpRequest(String baseUrl) generates only simple http-requests: final baseRequest = new http.Request(_getMethod(method), uri);. It would be nice to be able to create a multipart http-request.

http package docs

[Clancey]

So the hard part is making this generic enough. I am trying to think the right way to do it. Multipart is hard. If you were to subclass Request you could then add the specific properties you need for multipart. Then override the toHttpRequest and mix it in just like the doc says.