BibDocFile: more robust decompose_bibdocfile_url()

* Improves decompose_bibdocfile_url() function to be more robust with
  respect to uncommon characters in docnames.  (PR #1957)

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Reviewed-by: Tibor Simko <tibor.simko@cern.ch>