BibDocFile: fix version in register_download()

* Fixes exception raised when the file version passed to
  register_download() is not an integer (this can happen when
  the URL of the file does not explicitly contain the version
  number). (closes #1532)

Reviewed-by: Tibor Simko <tibor.simko@cern.ch>
BibField: legacy_export_as_marc escaping fix

* Fixes escaping in MARCXML cooking.  (closes #1509)

Co-authored-by: Tibor Simko <tibor.simko@cern.ch>
BibField: Python-2.4 compatibility fix

* Fixes Python-2.4 compatibility problem.  (closes #1533)

* Also standardizes the use of double quotes arrounf XML atributes.

Signed-off-by: Esteban J. G. Gabancho <esteban.gabancho@cern.ch>
Reviewed-by: Tibor Simko <tibor.simko@cern.ch>
Merge branch 'master' into next

* master:
  BibField: Python-2.4 compatibility fix
  BibField: legacy_export_as_marc escaping fix
  BibDocFile: fix version in register_download()