When compiling GROUP BY Item_ref instances are dereferenced in 
setup_copy_fields(), i.e. replaced with the corresponding Item_field 
(if they point to one) or Item_copy_string for the other cases.
Since the Item_ref (in the Item_field case) is no longer used the information
about the aliases stored in it is lost.   
Fixed by preserving the column, table and DB alias on dereferencing Item_ref