fix: paid amount in Sales Invoice POS return resets to 0

[ruchamahabal]

Steps to replicate:

1. Create a Sales Invoice for POS
2. Create a Return / Credit Note against it and Save.
3. On submit, the amount in the payments table gets reset to 0 because of this code.

erpnext/erpnext/public/js/controllers/taxes_and_totals.js

Lines 676 to 680 in a31731e

	this.frm.doc.payments.find(pay => {
	if (pay.default) {
	pay.amount = total_amount_to_pay;
	} else {
	pay.amount = 0.0

The default flag in the record gets reset here and is not set in this function:

erpnext/erpnext/controllers/taxes_and_totals.py

Line 639 in a31731e

self.doc.payments = []

Fix: Set default flag for payment method during return

[marination]

@ruchamahabal one small test for POS Invoice Credit Note ?

[ruchamahabal]

@marination There is already a test for pos returns

erpnext/erpnext/accounts/doctype/sales_invoice/test_sales_invoice.py

Lines 732 to 763 in 0ebace5

	def test_pos_returns_with_repayment(self):
	pos_profile = make_pos_profile()
	pos_profile.append('payments', {
	'default': 1,
	'mode_of_payment': 'Cash',
	'amount': 0.0
	})
	pos_profile.save()
	pos = create_sales_invoice(qty = 10, do_not_save=True)
	pos.is_pos = 1
	pos.pos_profile = pos_profile.name
	pos.append("payments", {'mode_of_payment': 'Bank Draft', 'account': '_Test Bank - _TC', 'amount': 500})
	pos.append("payments", {'mode_of_payment': 'Cash', 'account': 'Cash - _TC', 'amount': 500})
	pos.insert()
	pos.submit()
	pos_return = create_sales_invoice(is_return=1,
	return_against=pos.name, qty=-5, do_not_save=True)
	pos_return.is_pos = 1
	pos_return.pos_profile = pos_profile.name
	pos_return.insert()
	pos_return.submit()
	self.assertEqual(pos_return.get('payments')[0].amount, -500)
	pos_profile.payments = []
	pos_profile.save()

Looks like the test was not failing because this gets reset on the client side:

erpnext/erpnext/public/js/controllers/taxes_and_totals.js

Lines 676 to 680 in a31731e

	this.frm.doc.payments.find(pay => {
	if (pay.default) {
	pay.amount = total_amount_to_pay;
	} else {
	pay.amount = 0.0

[marination]

@ruchamahabal okay cool we can skip tests here