Change the default format to `DATE_ATOM` in `DateTimeFormatter`

[weierophinney]

Is there any reason why DateTime::ISO8601 is still used in DateTimeFormatter?

Taken from the docs:

DateTime::ISO8601 DATE_ISO8601 ISO-8601 (example: 2005-08-15T15:52:01+0000) Note: This format is not compatible with ISO-8601, but is left this way for backward compatibility reasons. Use DateTime::ATOM or DATE_ATOM for compatibility with ISO-8601 instead.

DateTime::ATOM DATE_ATOM Atom (example: 2005-08-15T15:52:01+00:00)

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Originally posted by @geerteltink at https://github.com/zendframework/zend-filter/issues/58

[weierophinney]

Most likely BC compliance: changing it would be a major BC break...

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Originally posted by @Ocramius at https://github.com/zendframework/zend-filter/issues/58#issuecomment-310582277

[gsteel]

Resolved via #160