Dev fix: When no ordering was aplied to the table, the next order would
throw an error
Dev: Change to how the init object is copied for multiple tables

Based on the discussion in the forms (
http://datatables.net/forums/discussion/18456 ) the fix from 8935b8a
wasn't good enough. What was happening was that the _save_data function
was doing a deep copy of all arrays (minus the data array) so the
default sort was stil applied.

The fix is to reinstate the _fnExtend function which does a jQuery like
deep extend on objects and a shallow extend on arrays. That is extended
slightly to allow array references to be broken on initialisation, to
resolve the issue 8935b8a attempted to (sorting array would be used for
two tables for example), by an optional parameter. This should resolve
the issue correctly this time!