Fix test for zero valued int and float attributes.

- Zero valued int and float attributes are now printed correctly;
- An _eventTypeClassFactory() object is now True even if
  it is only made of zero valued int and float attributes.
Merge remote-tracking branch 'upstream/master' into fix_zero_val_attribs
Fix for testcase described in #544.
Merge pull request #634 from claudiodsf/fix_zero_val_attribs

Fix for zero valued int and float attributes in core.event._eventTypeClassFactory()