[push-api] Can you change the value of PushSubscriptionOptions (#183)

When calling `subscribe()`, what happens if there is an active subscription with a different set of values for its `PushSubscriptionOptions`?  We can reject the call with `InvalidAccessError`,  `NoModificationAllowedError`, or even `InvalidStateError`.  Or we can create a new subscription with the new options.

I think that the current view is that rejection is cleanest (and leads to fewer surprises).  However, in the we will need to separately expose the current state of the options in the interests of transparency. 

This came up in #182.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/push-api/issues/183