Correct behavior of Order#outstanding_balance

[stewart]

This commit addresses an issue in Spree::Order#outstanding_balance, in
circumstances where an Order has had refunds applied.

The existing behaviour cancels out the amount of refunded payments,
resulting in a negative outstanding balance and an order state of
'credit_due', when in fact the order balance is zero. This is caused by
the calculation of payment_total already factoring in refunds.

Some more discussion about this behaviour can be found in the following
issues:

#1107
#1536

With these changes, outstanding_balance is now defined as
total - payment_total. This corrects the behaviour around refunds.

Tests have been updated accordingly, with some help from
DanielePalombo's commit:

c851404

[gmacdougall]

I confirmed the behaviour in a sandbox. The code is looking good as well.

Thanks Andrew. 👍

[jhawthorn]

This seems like a good change, but this reopens the issue which caused this in the first place, spree/spree#6229.

To summarize:

1. From a completed, paid, shipped order
2. Create an RMA
3. Create a Customer Return for that RMA.
4. Create a reimbursement

This leaves an order in the "balance due" state, which is wrong. We are not expecting more money from the customer since they have returned their item. Something needs to reflect this before this can be merged.

[jordan-brough]

Please also see #1254