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>