Impossible to use BaseObject#getListValue for users or groups properties

Reproduction steps:

• Create an xclass with a User list property
• Create a page with an xobject of the new xclass and put a value in the user list property
• Access programmatically to the xobject and try to list the values of the property by using
  BaseObject#getListValue

Expected result:

• the method should returns a list containing a single value

Obtained result:

• the call triggers a ClassCastException:

java.lang.ClassCastException: com.xpn.xwiki.objects.LargeStringProperty cannot be cast to com.xpn.xwiki.objects.ListProperty
	at com.xpn.xwiki.objects.BaseCollection.getListValue(BaseCollection.java:490)