Fix vocabulary entry update when id contains at least 2 consecutive slashes

Steps to reproduce:

1. log in as Administrator to WebUI
2. navigate to Administration > Vocabularies
3. select "predicates"
4. edit the first entry
5. change the label and save
6. observe the message : ERROR: entry not found

The root cause of thje bug is that

http://purl.org/dc/terms/References

is transformed into

http%3A/purl.org/dc/terms/References

in the URL of the PUT request => a slash is missing

The issue can be easily worked around by manually running the corresponding curl request where the id would have been fixed with the correct number of consecutive slashes.