Fix #3984 - Fix broken check for drupal_views_user_enum

The current version in master will get "Check failed: ArgumentError wrong number of arguments (0 for 1)". This patches that (also see #3984).

## IMPORTANT

Before you land this pull request, **make sure #3988 is landed first** Because that PR uses the broken version of drupal_views_user_enum for testing.

## To verify

- [ ] You can set up a real drupal server, or ```ruby -run -e httpd . -p 8181``` for a fake one, which should be enough
- [ ] ```use auxiliary/scanner/http/drupal_views_user_enum```
- [ ] ```set rhosts [ip]```
- [ ] ```set rport [port]``` if you're using the fake server
- [ ] ```check```
- [ ] Instead of an error, you should see a check status message. Something like "The target is not exploitable".
- [ ] If you're using the fake server like the one above, you should see this in your access log: "GET /?q=admin/views/ajax/autocomplete/user/"
I could be missing something here, but why use an instance variable?  It isn't used anywhere that I can see, or at least not intentionally.