Strict mode for Date validator

[michalbundyra]

Input must be always in the defined format and must be the same as output of format method of DateTime.

Strict mode is set to false by default to keep BC.

Currently to the validator we can provide integer, double, array, string or DateTimeInterface value. In my opinion as validator has set format we should check if the input value has exactly the same format as set. In strict mode we are comparing if:

$input === DateTime::createFromFormat($format, $input)->format($format);

Resolve #33 Fix zendframework/zendframework#6407

• [x] Are you creating a new feature?
  • [x] Why is the new feature needed? What purpose does it serve?
  • [x] How will users use the new feature?
  • [x] Base your feature on the develop branch, and submit against that branch.
  • [x] Add only one feature per pull request; split multiple features over multiple pull requests
  • [x] Add tests for the new feature.
  • [ ] Add documentation for the new feature.
  • [ ] Add a CHANGELOG.md entry for the new feature.

I would see strict mode by default enabled in v3, or ONLY strict mode in v3. Thoughts?

/cc @svycka @gianarb