Re: [whatwg/fetch] Is a missing HeadersInit treated differently from an empty one? (#479)

Well, it's a general API design decision.  For dictionaries, IDL explicitly has identical behavior for `null`, `undefined`, and `{}` as operation arguments.  It does not have that for dictionaries as dictionary members, but I think it should; that's what the discussion in https://github.com/heycam/webidl/issues/76 is about.

For records, we made the behavior match dictionaries, because I guess people felt conceptually they were kind of the same (maybe because they were still thinking of the "half-open dictionary" case?).  Note that this doesn't match what I had done with MozMap in Gecko, where I made it much more like a sequence than like a dictionary in terms of all this stuff (e.g. you can't pass undefined or null for a MozMap argument, and MozMap isn't forced to be an optional arg in trailing position, etc).

It's possible that speccing record to be like dictionary was just a mistake.  After all, `{}` as a dictionary just means "no values provided for any of the members", so it makes sense to give `null` and `undefined` the same behavior.  But for a record `{}` is more like an explicit "the list is empty" which maybe shouldn't mean the same thing as "no list provided".

So @jyasskin, @domenic, what behavior do we want here?

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