jsonalchemy: hotfix for optional fields

* Fixes an unexpected behaviour which happen when a field in the
  conversion sheet is not present in the marcxml (e.g. email).  When
  marcxml is transformed to dict everything is fine, but when it is
  converted back to marcxml, it will get into an infinite loop because
  the field is not present in the dict. Hence it will try to eval the
  field and it will end in an infinite import loop.

Signed-off-by: Guillaume Lastecoueres <guillaume.lastecoueres@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>
jsonalchemy: unit tests for module import fix

* Adds a field to fields.cfg, which is a name of module.

* Adds a test that will try to convert a marcxml to json
  and then from json to marcxml. In this test email is
  a field which is not present in the marcxml, and then
  bibfield will try to import it like a module.

Signed-off-by: Guillaume Lastecoueres <guillaume.lastecoueres@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>