Fixed issue with builtin filtering.

The superclass (which ultimately inherits from 'Handler')
checks against any registered filters, before allowing a
logging record. Nose's logging capture plugin should do the
same, because these filters are allowed to modify the record.

And it can easily happen that the logging format requires
this modification (e.g. add an attribute).
Merge pull request #727 from malthe/fix-builtin-filtering

Fixed issue with builtin filtering.