jsonalchemy: bug fixes
- Fix record tests when using '001' marc tag.
- Implement _prepare_blob method for json_reader to use the model field names when using a JSON input.
- Fix model inheritance when overriding fields.
- Fix validation when using models and override fields.
- Enhance creation_date field definition.
- PEP8 fixes.
Signed-off-by: Esteban J. G. Gabancho <egabancho@gmail.com>