Listing Users

I was getting the following PHP error when creating an issue. This is a fix for it.
```
PHP Warning:  Invalid argument supplied for foreach() in /home/jbutz/workspace/redmine/vendor/kbsali/redmine-api/lib/Redmine/Api/User.php on line 42
PHP Warning:  Invalid argument supplied for foreach() in /home/jbutz/workspace/redmine/vendor/kbsali/redmine-api/lib/Redmine/Api/User.php on line 42
```