Add FORMAT_RFC3339_STRICT.

[krisfremen]

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• [x] 🧪 Added tests for changed code.
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Description of Changes

Adds a FORMAT variable for a more strict RFC3339 where the RFC called for a space for readability and is used by FORMAT_RFC3339, FORMAT_RFC3339_STRICT uses the T separator to adhere with ISO8601 spec.

RFC Link

      NOTE: ISO 8601 defines date and time separated by "T".
      Applications using this syntax may choose, for the sake of
      readability, to specify a full-date and full-time separated by
      (say) a space character.

Closes #1138

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (403c29f) to head (f13d917). Report is 11 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1201 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 10 10 Lines 2324 2335 +11 Branches 357 357 ========================================= + Hits 2324 2335 +11 ```

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