REST attribute value for "separator" is not correct for most of the responses to a request

The model returned by REST for classes is not correct regarding the attribute "separator".
For example for the object "XWiki.XWikiGlobalRights" we've got :
<attribute name="separator" value=" "/>
But when you create an object for this property, for example "levels", we are sending a list with "," as a separator.

Example :
Content
curl -u admin:admin \
-X POST \
-H "Content-type: application/xml" \
-H "Accept: application/xml" \
-d "@test9.xml" \
http://localhost:8080/xwiki/rest/wikis/xwiki/spaces/Projet%20Ocean/pages/WebPreferences/objects

File "test9.xml" :
<object xmlns="http://www.xwiki.org">
<className>XWiki.XWikiGlobalRights</className>
<property name="allow"><value>1</value></property>
<property name="groups"><value>XWiki.GroupeUn</value></property>
<property name="levels"><value>login,view,edit,delete,comment,script,programming</value></property>
<property name="users"><value></value></property>
</object>