[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        | ~
I used it first but `formatValue()` will output `"3"` instead of `3` for example in the violation message for scalar values.