User reference in group stored with full reference even if it's not needed

Step to reproduce:

• Create a sub-wiki
• Call org.xwiki.contrib.authentication.internal.DefaultUserManager#addToGroup from the sub-wiki with a group on the main wiki and with a user reference which is in the main wiki.

Expected:
As the group and the user are in the main wiki the reference stored would be something like XWiki.Userx.

Currently:
As the serializer have in the context the sub-wiki the user reference contains the wiki part and so it's stored like xwiki:XWiki.Userx. And even worse if the context changed, we can have the user duplicated because have the variant stored into the XWikiGroup object.