Ticket/22502

Nixed microseconds on datetime BoundField values generated from a callable default. Fixed [Django ticket 22502](https://code.djangoproject.com/ticket/22502).

Explanation: If a callable default creates an initial datetime value with microseconds, those microseconds will be reflected in the hidden input used to track the initial value - but *not* in the datetime field presented to the user. That means that the field will *always* show up as changed when the user submits the form. This is particularly relevant for extra forms on a formset (which, due to this bug, will run validation and potentially be saved even though if the user hasn't touched them).

The only potential edge case where I could see this causing a problem would be if someone wrote a datetime field which *had* microsecond support. I don't think that's something folks will want to do, though.