copyDocument should not copy the custom mapping of the document xClass

The rational is that copying an "internal" mapping would probably never work since the hibernate mapping stay unchanged, and that copying a dynamic custom mapping would cause 2 separate class to be mapped the same.

The most common example is copying XWiki.XWikiPreferences, which cause the XWiki.XWikiPreferences class to be cloned. This new copy of the class should not be set to "internal" custom mapping, or it will be unusable.