BibDocFile: fix error in calling register_download

* When BibDoc.register_download() was called by WebInterfaceFilesPages
  the version parameter was not correctly passed when the version
  was not explicitly set in the URL argument.  This is now fixed.
  (closes #1311)

Reported-by: Johnny Mariéthoz <johnny.mariethoz@rero.ch>
Reviewed-by: Tibor Simko <tibor.simko@cern.ch>