PIM-2355 Fixed uniqueness validation constraint with mongodb

| Q                   | A
| ------------------- | ---
| Bug fix?            | yes
| New feature?        | no
| BC breaks?          | no
| Tests pass?         | no
| Scenarios pass?     | yes
| Checkstyle issues?  | no
| Changelog updated?  | no
| Fixed tickets       | PIM-2355
| Doc PR              |

@nidup I'd like to review with you the input data of `DateFilter::addFieldFilter()`. It seems that `$value` is sometimes a `\DateTime`, sometimes a string (eg.: 2012-04-17) and sometimes a timestamp.