Fixed #20292: Pass datetime objects (not formatted dates) as params to Oracle

This seems worthwhile in its own right, but also works around an Oracle
bug (in versions 10 -- 11.1) where the use of Unicode would reset the
date/time formats, causing ORA-01843 errors.

Thanks Trac users CarstenF for the report, jtiai for the initial patch,
and everyone who contributed to the discussion on the ticket.

tests/model_fields/models.py

@@ -76,6 +76,12 @@ class BooleanModel(models.Model):
     string = models.CharField(max_length=10, default='abc')
 
 
+class DateTimeModel(models.Model):
+    d = models.DateField()
+    dt = models.DateTimeField()
+    t = models.TimeField()
+
+
 class PrimaryKeyCharModel(models.Model):
     string = models.CharField(max_length=10, primary_key=True)