FIX Defensively copy mocked datetime

[dmipeck]

Datetime::now() currently returns a reference to self::$mock_now if a mock time has been set. This means that if one calls Datetime::now()->Modify(...) the next time Datetime::now() is called it will return the modified datetime rather than the original. This PR has Datetime::now() return a defensive copy of self::$mock_now so that the mock time cannot be accidentally modified and can only be changed via Datetime::set_mock_now()

[kinglozzer]

I’ve encountered this issue too, this should save having to do a bunch of clone-ing in user code