Christoph Büscher 0d81dee013 Fix key_as_string for date histogram and epoch_millis/epoch_second format
When doing a `date_histogram` aggregation with `"format":"epoch_millis"` or
`"format" : "epoch_second"` and using a time zone other than UTC, the
`key_as_string` ouput in the response does not reflect the UTC timestamp that is
used as the key. This happens because when applying the `time_zone` in
DocValueFormat.DateTime to an epoch-based formatter, this adds the time zone
offset to the value being formated. Instead we should adjust the added display
offset to get back the utc instance in EpochTimePrinter.

Closes #19038
2016-06-29 19:18:12 +02:00
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