Object class is lost in history when the document is saved before the xclass it depends on

For example you can reproduce that by copying between wikis a document contains an object with a class that does not exist on target wiki.

The only way to fix it properly is to refactor BaseObject to contains the BaseClass it's been created from instead of just the class reference.

This is not too critical with the bulletproofing added by XWIKI-10079 but still not very nice since the class may still not exist when loading the history or the class could have changed a lot.