The owner of a subwiki is automatically added to the Admin group of that subwiki

Steps to reproduce:

1. Login with Admin
2. Create 2 global users: U1 & U2
3. Go to Administration -> Wikis and install the Wiki Template
4. Create a new subwiki and in step 2 set U1 as the owner and U2 as a member
5. Hover over Wiki and click "Administer Wiki"
6. Click Wiki
7. Change the subwiki's owner to U2
8. Click Save
9. Click Groups
10. Click XWikiAdminGroup

You'll see both the previous and the current users (U1 & U2) listed.