Core ping sends localized date

[st3fan]

The created field contains a localized date. It should always be of the form YYYY-MM-DD in GMT instead.

locale,created,app_name,os
ar-SA,٢٠١٨-٠٤-١٧,Focus,iOS
ar-SA,٢٠١٨-٠٤-١٨,Focus,iOS
ar-IQ,٢٠١٨-٠٤-١٨,Focus,iOS

[st3fan]

I think the CreatedDateMeasurement should be using a ISO8601DateFormatter instead of the `DateFormatter'.

[justindarc]

This should've been fixed in PR #82 and #83... let's verify that those pings with invalid dates are from versions older than December 8, 2017.