Re: [fetch] OpenEndedDictionary clarification (#164)

@annevk Oh, and the other slightly open question is what the behavior should actually be.  As in, Domenic's semantics involve calling the actual "append" method (which may be overridden by the web page, subclassed, etc) whereas the current spec language is calling the internal "append" method instead.  This difference is super-important: if you're going to call the internal method, then of course you want to do the type coercion before you do that, but if you're going to call `this.append` then of course you _don't_ want to do the type coercion before doing so.

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