Re: [fetch] OpenEndedDictionary clarification (#164)

The idea of `OpenEndedDictionary`, in general, is to capture the idea of something like an options object (i.e. dictionary) but where the set of key names is not known ahead of time (but the keys are known to be strings, and the values are all known to be some type, possibly a union type).

There are some specs that want this sort of thing (the `Headers` spec, some of the animations API stuff, though the latter has some fixed set of keys _and_ additional open-ended ones).

---
Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/164#issuecomment-160828155