[suggestion] W3C Headers
Just wanted to see if you want to use my lib i created yesterday: [fetch-headers](https://www.npmjs.com/package/fetch-headers)

It has some good parts like for...of iterator, case insensitive, getAll, keys, values
...Follows the W3C spec

example:

``` javascript
header = new Headers({
  accept: 'application/json']
})
header.append('accept': 'text/plain')

console.log([...headers]) // [["accept","application/json"],["accept","text/plain"]]
```
