[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        | ~
No reasons! Maybe it's the way to go to use `formatValue()`, then I will update the code. @stof @webmozart any thoughts please?