Support `FormData`

Posting FormData can be done using something like:

``` js
var fd = new FormData()
fd.append('image', file)
var req = new XMLHttpRequest()
req.onload = function () {
callback(null, req.responseText)
}
req.onerror = callback
req.open('post', 'https://api.imgur.com:443/3/image.json')
req.setRequestHeader('Authorization', 'Client-ID ' + clientID)
req.send(fd)
```

However, I can see know way of handling `FormData` using the built in `http` module.

(see https://github.com/substack/node-browserify/issues/439)

Start by reading the built-in http module's request and body-handling entry points, then compare them with the FormData and XMLHttpRequest example in the issue. Done should mean FormData can be posted through the module while existing request behavior continues to work; the issue does not name a file or test to run.

Vom Indexierungsmodell aus dem Issue-Text verfasst.