@sescandell in the FieldGuesser [getFilterType](https://github.com/symfony2admingenerator/AdmingeneratorGeneratorBundle/blob/master/Guesser/DoctrineORMFieldGuesser.php#L129) if key is not found, it will fallback to `getFormType`

I've added these (without a default value) to avoid the "Unknown configuration option" error, but they should not be set by default (becouse they have no default value).

This should result in adding `TIMESTAMP` to `filter_types` only if it's set by user.