[patch] models.DateField should allow string lookups

[Esaj]

At the moment DateField.get_db_prep_lookup assumes that values passed to it are datetime objects when doing certain lookup types.

This patch falls back to using str(value), which means that various date lookup queries can work in the admin changelist.

[Malcolm Tredinnick]

See also #2211. Not sure which patch is better at the moment.

[Malcolm Tredinnick]

(In [3223]) Fixed #1754, #2211, #2192 -- allow date filtering comparisons to use strings as
well as date objects. Fixed a couple of admin crashes as well.