Wrong header instance

[jimmywarting]

Found this minor issue while testing

Chrome

h = new Headers({'x-foo': 'x-bar'})
res = new Response('x', {headers: h})

console.log(h === res.headers) // false

Polyfill

h = new Headers({'x-foo': 'x-bar'})
res = new Response('x', {headers: h})

console.log(h === res.headers) // true

Maybe it should be simplified from:

this.headers = options.headers instanceof Headers ? options.headers : new Headers(options.headers)

To just:

this.headers = new Headers(options.headers || {})

[jimmywarting]

test:

  // #416
  test('creates a new instanceof headers when constructing', function() {
    var h = new Headers({'x-foo': 'x-bar'})
    var res = new Response('x', {headers: h})

    assert.equal(h === res.headers, false)
  })