Fixed save payment_method in admin edit order.

Based on issue #2361 I began to investigate the difficulty and found that "payment_method" was not being saved.

After my changes I realized several tests and always saved correctly.

The problem also seems to be happening with the "shipping_method". If no fix for this day maybe I correct the night.

![image](https://cloud.githubusercontent.com/assets/1612563/5223003/f1d0b006-7693-11e4-8f7e-8b179df5a7e9.png)