Remove unintended side-effect

create_user_address modified the passed dictionary, which is
undocumented, and as far as I'm aware, unintended.
This lead to altering session data.

The problem was spotted because I call place_order multiple times in one
checkout process, and the UserAddress form wouldn't accept the user_id
keyword parameter that it was passed on the second call of place_order.

It is fixed by copying the passed dictionary and only altering the latter.
Merge pull request #528 from maikhoepfel/fix/shippingsideeffect

Remove unintended side-effect