Filter_in fix

[nursix]

Setting the default for a datetime field to datetime.datetime.utcnow (a built-in function) would crash upon insert, due to the mandatory filter_in for that field type.

Either callable defaults are in fact called before filter_in, or filter_in skips them altogether. The difference is that in the former approach, the result of the callable would still be converted with _todatetime, whereas the latter approach simply sends the datetime object for conversion by the representer.

It seems more consistent to also pass the output of a callable through filter_in (note: the output - not the callable, as it were), however, it is a bit confusing why the default _filter_in produces a string for a datetime object if the representer can perfectly handle a datetime object - including the (theoretically dialect-specific) dt_sep?