[fetch] Header guards (#182)

As part of [fetch-engine](https://github.com/phuu/fetch-engine), @ahume and I have run into the [guard][concept-guard] state of the [Headers][headers-class] object. We have some questions:

- How is the guard initially set? There doesn't seem to be a parameter for it.
- What does the external API for getting and setting the Headers' guard value look like?

[headers-class]: https://fetch.spec.whatwg.org/#headers-class
[concept-guard]: https://fetch.spec.whatwg.org/#concept-headers-guard

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/182