Add support for property converters to findBy.

[TSchuermans]

If a property has a DateTime converter annotation, it should be possible to use a DateTime object in findBy.
AFAIK you can only use the unix timestamp or the long unix timestamp in findBy.

It would be useful to be able to do:
$repository->findBy(['property' => new \DateTime()])

This involves parsing the metadata of the class, so it might not be that optimal.
The other approach could be to convert \DateTime values to their unix timestamp, but then you risk a mismatch if the long timestamp format is used.