Fix vocabulary management when the entry key starts with @

Steps to reproduce:

1. log in as an Administrator
2. go to the Vocabularies page
3. select "continent"
4. add an entry whose key is @robase
5. click on Delete icon to delete the new entry
6. observe that an error is display in the bottom left corner

curl -X DELETE'http://localhost:8080/nuxeo/api/v1/directory/continent/@robase?pageSize=0'

returns the response

{
  "entity-type": "exception",
  "status": 404,
  "message": "Service robase not found for object: /nuxeo/site/api/v1/directory/continent of type directoryObject"
}

Replacing @ by %40 in the URL fixes the problem