- add fix method for BTree if its _folder_handler is set to string
 - add tests:
   - to check if it is possible to recover functionality of isBTree and isHBTree methods using _fixFolderHandler
   - that BTree after adding content is still BTree while using isBTree (same for HBTree)


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@26327 20353a03-c40f-0410-a6d1-a30d3c3de9de