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>