OF-1094: Allow reset of UserProvider

When the property provider.user.className is set, a new UserProvider will be initialized.
Openfire should revert back to the default provider when that property is deleted.

src/java/org/jivesoftware/openfire/user/UserManager.java

@@ -107,7 +107,9 @@ public class UserManager implements IQResultListener {
 
             @Override
             public void propertyDeleted(String property, Map params) {
-                //Ignore
+                if ("provider.user.className".equals(property)) {
+                    initProvider();
+                }
             }
 
             @Override