Add discount_rate to line_items and include in calculations.

[latentflip]

Fixes #56.

@waynerobinson this isn't perfect yet, particularly it's untested, but I wanted to open it to start the discussion anyway.

It's not clear to me why we manually calculate the line item amount, when xero appears to return it from the API. line_item.rb/33 Is there a reason why we recalculate it?

[waynerobinson]

This is mainly for creating new invoices however, the library needs a better way of determining whether the entry has been loaded (and not changed) from Xero or whether it has been created/updated.

Also, I'm not sure if it is strictly necessary to include the line total any more for creates/updates… this needs more testing.

[latentflip]

This is buggy, definitely don't merge it yet 😄

[nnc]

@waynerobinson Xero docs now clearly state that LineAmount is optional, so this should be revisited as it still doesn't work for line items with discounts.

@latentflip do you remember which bugs you ran into with this patch?

[mphalliday]

Is there any update on this bug? We are having issues when editing an inclusive invoice that has line item discounts.

I see this has been an issue with Xeroizer since 2013. There is also this PR #217 which has been closed. Not sure why. I'm happy to help out and do a PR to get this fixed.

[rjaus]

Looks like this has been added now via this commit 3e0248a