[Validator] Convert objects to string in comparison validators

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

In the [latest merge from 2.3 into 2.4](/symfony/symfony/commit/3bed1b7988e94a897a64c6a2ad3bf70bde9005c1), the changes from 6cf5e0812e6f20d60acbc0324abf96475e89b6ef in 2.4 got lost. This PR brings back these changes and backports them to 2.3.

The change is BC, because the former value `true` of the `$prettyDateTime` will be cast to `1`, which corresponds to the `PRETTY_DATE` format constant.
should this not also support http://php.net/manual/de/class.datetimeinterface.php from php 5.5