Re: [fetch] Header guards (#182)

I ended up implementing subtypes of the Headers class for each of the guard values. RequestHeaders, ResponseHeaders, etc... which set a private guard property in their constructors. Seems like a clean approach, but we'll see how this pans out as we implement Request/Response. The base class still relies on the internal guard property, but the APIs never have to worry about reading or writing these.

@phuu made the point that eg. immutable headers throws a TypeError if you try to write to it. Could that be an indication that types are a good way to go here?

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