Added FormData support

[liamcurry]

This PR adds basic FormData support using the form-data module (same module that request uses). This may not cover all cases, but it's working in my project so I thought I'd share.

Here's an example of how you'd use this to polyfill FormData in Node:

import XMLHttpRequest from 'xhr2';
global.XMLHttpRequest = XMLHttpRequest;
global.FormData = XMLHttpRequest.FormData;

[pwnall]

Sorry for the late response. This is a feature I'd be interested in, and I like that you followed the choices in request. Can you please add tests?

[mebibou]

This doesn't work in this case:

const data = new FormData();
data.append('a', 1);
data.append('b', 1);

In this case, it adds some function in the streams of the FormData forcing the data.on('data') to be completely called asynchronously. In other words, you have to liste to data.on('end') callback function to be sure to have the complete body.