Fix retrieval of personal workspace for user with special characters

Steps to reproduce:

1. Log in and go to the JSF UI
2. Go to Admin > Users & Groups
3. Click the "Create a New User" button
4. Create a first user with username a-b and don't check the "Create another" box
5. Click on the "Back to the List" button
6. Click the "Create a New User" button
7. Create a first user with username a_b

Now the following actions will fail:

• GET the path endpoint with the theoritical path of a_b workspace

  http://localhost:8080/nuxeo/site/api/v1/path/default-domain/UserWorkspaces/a_b
  

  returns a 404 error

  {"entity-type":"exception","status":404,"message":"/default-domain/UserWorkspaces/a_b"}
  

• cURL command to get the user workspaces from the GetUserWorkspace' operation

  curl -X POST 'http://localhost:8080/nuxeo/site/automation/User.GetUserWorkspace' -u a_b:password -H 'content-type: application/json' -H 'X-NXproperties: *' -H 'X-NXRepository: default' -H 'X-NXVoidOperation: false' -d '{"params":{},"context":{}}'
  

  => returns the personal workspace for a-b

  {"entity-type":"document","repository":"default","uid":"80172e2a-a9b2-4fed-86dc-e83510855485","path":"/default-domain/UserWorkspaces/a-b", ...
  

AbstractUserWorkspaceImpl#getCandidateUserWorkspaceNames produces the alternative names for the personal workpace

    protected PathRef getExistingUserWorkspace(CoreSession session, PathRef rootref, Principal principal,
            String username) {
        PathRef freeRef = null;
        for (String name : getCandidateUserWorkspaceNames(username)) {
            PathRef ref = new PathRef(rootref, name);
            if (session.exists(ref)
                    && session.hasPermission(session.getPrincipal(), ref, SecurityConstants.EVERYTHING)) {
                return ref;
            }

=> this permission check uses a "system" user