Assume the default time to freeze is "now".
Assume the default time to freeze is "now".

This is useful to avoid having to tediously pass in an initial time,
allowing for cleaner tests like:

    def test_foo(self):
        with freeze_time() as t:
            spam()
            t.tick()
            eggs()

instead of:

    def test_foo(self):
        dt = datetime.datetime.utcnow()
        with freeze_time(dt) as t:
            spam()
            t.tick()
            eggs()

Signed-off-by: Chris Lamb <711c73f64afdce07b7e38039a96d2224209e9a6c@chris-lamb.co.uk>

Affirm/freezegun,Sun77789/freezegun,spulec/freezegun