Fix no auto line voucher.

Voucher.line are deleted when amount of money changes we have to copy
the old line before calling super classes. Then returning the copy of
old lines.
It was used to pass the context. The context contains the line_cr_ids and line_dr_ids of the actual table. But they are later unlinked from the model and deleted... 

How account_voucher works is problematic. It makes the button "discard" unusable.