Global user may loose local subwiki rights received via global groups

Steps to reproduce:

• Create a global user test in a global group test
• Create a subwiki, and put the global group test as a member of the local XWikiAdminGroup
• At this stage, user test is admin of the subwiki
• Save the XWiki.Preferences page of the subwiki (should clear the subwiki cache), or change the members of the subwiki XWikiAdminGroup
• Now the test user has no more admin rights on the subwiki (fully if cache cleared, or only new evaluation if just admin group changed)