Fix duplicate address records being created during the checkout process.

The bug happens when a user fills out their address details in the address step, continues to the delivery step, goes back to the address step, and continues to the delivery step again. When the user continues the checkout process the addresses are created as new records in the database rather than being updated. This is because the id parameter isn't permitted by the controller, without which the existing records cannot be updated, resulting in new records being created for every change to the order addresses.
adjustment_state (icon) returns 'fa fa-lock'

The adjustment_state helper returned `<span class="lock"></span>` which is not the correct html for an icon.

I edited the helper and it returns the right classname now.
So the icon is visible again.
Merge pull request #4900 from reinaris/patch-3

adjustment_state (icon) returns 'fa fa-lock'
Merge pull request #4506 from MattFenelon/address_checkout_bug

Fixed duplicate address records being created during the checkout process
fix address spec
Call to_s instead of underscore on payment_method_id

Cant think of a reason it were always calling underscore on an id. That
line seems to be around since forever
Rename return_authorization_inventory_units to return_items

Shortening the name of return_authorization_inventory_units since it was becoming a nuisance.
Fixes #4920