[heycam/webidl] Clarify whether a dictionary member set to `undefined` is "present" (#124)

https://heycam.github.io/webidl/#dfn-present says, "When specified in the dictionary value, a dictionary member is said to be present, otherwise it is not present."

It is not immediately clear whether a value set to `undefined` should be considered "specified in the dictionary value" and thus present. The same issue may apply for `null`.

It appears (i.e. https://github.com/w3c/encrypted-media/issues/178#issuecomment-216124376) at least some implementations may consider `dictionaryValue:undefined` to be *not* specified while actually tracking the presence of the value (i.e. in dev tools).

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