Set write-off as default when paying exact amount of invoice

When paying the exact remaining amount of an invoice with different currency,
it should normally be a payment with write-off for exchange rate.

Closes #11821

modules/account_invoice/invoice.py

@@ -3312,7 +3312,7 @@ class PayInvoice(Wizard):
         default['currency_writeoff'] = invoice.company.currency.id
         default['invoice'] = invoice.id
 
-        if (amount_invoice > invoice.amount_to_pay
+        if (amount_invoice >= invoice.amount_to_pay
                 or invoice.company.currency.is_zero(amount)):
             default['type'] = 'writeoff'
         return default

modules/account_invoice/tests/scenario_invoice_alternate_currency.rst

@@ -165,7 +165,8 @@ Pay the invoice with rate change::
     >>> pay.form.payment_method = payment_method
     >>> pay.form.date = tomorrow
     >>> pay.execute('choice')
-    >>> pay.form.type = 'writeoff'
+    >>> pay.form.type
+    'writeoff'
     >>> pay.form.writeoff = writeoff
     >>> pay.form.amount
     Decimal('460.00')