Replace outdated DATE_ISO8601 in favor of DATE_ATOM

[jorgsowa]

This PR aims to replace all of the usage of DateTime::ISO8601 and DATE_ISO8601 constants in favor of correct format of DATE_ATOM. Next step would be to deprecate this constant with acceptance of the community. But I hope PR correcting all the tests doesn't need the formal way.

In some tests I have added missing DateTime constants like DATE_ISO8601_EXPANDED, DATE_RFC3339_EXTENDED and DATE_RFC7231.

One tests has been skipped (bug71062.phpt), because it clearly tests this constant.

[kocsismate]

But I hope PR correcting all the tests doesn't need the formal way.

We cannot get rid of tests testing DATE_ISO8601 until it's completely removed. Even though it may not be the "correct" format, it's still widely used.

In some tests I have added missing DateTime constants like DATE_ISO8601_EXPANDED, DATE_RFC3339_EXTENDED and DATE_RFC7231.

I suggest that you should separate these changes into their own PR if you want them get merged. The rest of the PR won't be accepted until an RFC passes.

[jorgsowa]

Thanks @kocsismate for answer. I'm actually not removing any test. I'm just using more widely used date format constant in those tests. There is only one test testing explicitly DATE_ISO8601 constant and I left it untouched (bug71062.phpt). My PR is kind of improvement in way of using iso8601 compliant format.