fix shipping rates refresh losing current rate

The admin order creating seems to replace all the shipping rates and loses the current selected rate.
Seems to be affected be the change made here:
https://github.com/spree/spree/commit/8f3c82c04b7c3735e9e6d523b366ea3c2dea34a7
So this brings back the 'store and check' of shipping_method.id

To reproduce:
- Log in as admin
- Go to orders, create order
- add item
- change shipping rate => changes don't stick, always resets to default

Affects 2-0-stable as well as 2-1-stable and master