Z and ZZ tokens should return Z when UTC mode

[maggiepint]

Pull request #3098 modified the default format function to return Z as the offset when in UTC mode, pursuant to the ISO8601 specification.

This only occurs when using the default format. As discussed in the pull request itself, it probably makes sense to either change the behavior of the Z and ZZ tokens to work this way as well, or to make a new token that does.

[twentyonesquared]

Any progress on this?
We're working with an API that expects dateTimes in ISO8601 format (utc). That is 2016-06-10T16:47:00Z but moment returns 2016-06-10T16:47:00+00:00

[billymoon]

+1