Field set to Now() expression doesn't appear as dirty.

[LincolnPuzey]

Originally reported by @lestatcheb in #117

Test case that currently fails:

class DatetimeModelStandard(DirtyFieldsMixin, models.Model):
    datetime_field = models.DateTimeField(blank=True, null=True)

@pytest.mark.django_db
def test_dirty_fields_field_now():
    tm = DatetimeModelStandard.objects.create()

    assert tm.get_dirty_fields() == {}

    tm.datetime_field = Now()

    assert tm.get_dirty_fields() == {'datetime_field': None}

E       AssertionError: assert {} == {'datetime_field': None}
E         Right contains 1 more item:
E         {'datetime_field': None}
E         Full diff:
E         - {'datetime_field': None}
E         + {}

I think this is a case that should pass. The Now() expression represents a value that is different to the database, so the field is dirty.

Ideally a solution should work for the different database expression objects available, not just Now().