Fix Headers bug in IE11

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When calling new Headers(undefined) in IE11 an Invalid argument exception is thrown.
With this in place, we'll call it with an empty object, resulting in an empty Headers object.

TODO:

• Update CHANGELOG.md with your change (include reference to issue & this PR)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

Edit:

I mistakenly wrote null, where I meant to write undefined.

[Multiply]

I haven't added tests for this, as I am unsure if you've got any IE11 targets in your test suites.

Edit:
This also affects Edge, it seems.

In IE11, it's running using the default polyfills.