[Validator] make DateTime objects represented as strings in the violation message.
bug #12437 [2.6] [Validator] make DateTime objects represented as strings in the violation message (hhamon)

This PR was submitted for the master branch but it was merged into the 2.6 branch instead (closes #12437).

Discussion
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[2.6] [Validator] make DateTime objects represented as strings in the violation message

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | ~
| License       | MIT
| Doc PR        | ~

Commits
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b753218 [Validator] make DateTime objects represented as strings in the violation message.