[FIX]Delivery: carrier information not propagated from sale order to pickings (Case: ref 595240)

closes lp:1192115
[FIX] account_voucher: better pay invoice wizard

If the writeoff_amount is above 0, the fields writeoff_acc_id can be displayed and is required depending of the payment option. If the writeoff_amount is reset to 0, this field should no longer be required.
Merge pull request #415 from gurneyalex/7.0-fix_lp1192115-595240-opw-skh

[FIX] delivery: do not remove the delivery method on the sale.order when adding it as an order line (useful information)
[FIX] fields.py - avoid dict comprehension inside a dict comprehension
[MERGE] forward port of branch 7.0 up to 100eba8