Normalise min/max-options for sfValidatorDate

[nederdirk]

The min/max options of sfValidatorDate were not normalised in the same way as the input, resulting in the following incorrect behaviour:

$validator = new sfValidatorDateTime([
  'min' => '2018-12-06T09:00:00+00:00',
]);
$validator->clean('2018-12-06T09:59:59+01:00'); // this should throw, because 09:59+01:00 is one minute before 09:00+00:00, but it doesn't

This merge request adds tests for this situation and fixes this behaviour

[alquerci]

Hello @nederdirk,

Here is not an issue with the time zone but the way where the date time are compared.

The proper way to do it is to compare timestamps.

As the timestamp does not depend on the timezone then all issue will be resolved there.