Allow FormData constructor to accept array or object too

[jimmywarting]

Like the URLSearchParams allow the FormData constructor able to accept a iterable array or object
instead of having to manually append everything

Current way of solving things:

var data = ['foo', 'bar']
var data = {foo: 'bar'}

var fd = new FormData()

for (let [key, val] of new URLSearchParams(data)) {
  fd.append(key, val)
}

// or one-liner 
new URLSearchParams(data).forEach(args => fd.append(...args))

Suggested way

var data = ['foo', 'bar']
var data = {foo: 'bar'}

var fd = new FormData(data)

[annevk]

This doesn't seem entirely unreasonable, but please file it at https://github.com/whatwg/xhr/issues/new as that's where FormData is defined. Thanks!