[fetch] OpenEndedDictionary clarification (#164)

In the spec we are told:

OpenEndedDictionary<T> is a future IDL construct. Expect it to be used as such:

```javascript
var meta = { "Content-Type": "text/xml", "Breaking-Bad": "<3" }
new Headers(meta)
```

Sadly the currently liked IDL draft does not specify what this means.  Can someone clarify?

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