Play nice with moment-timezone

[butterflyhug]

Currently, moment-timezone doesn't work properly on frozen moments because its updateOffset implementation assumes that it is using the mutable API.

In moment/moment-rfcs#2, I proposed automatically thawing the moment before updateOffset and then re-freezing afterward (or copying the changed properties when updateOffset is called during the constructor). I'm not sure how a plugin would get access to do that, but I'll leave this issue open in case other folks have better ideas.