[FIX] account, purchase: payment term in vendor bill

When creating a vendor bill from a PO, the payment term configured in
the PO was lost due to function _onchange_partner_id.

When creating a vendor bill, the field payment term was invisibile if
there was no payment term set on the vendor.

opw:744714

addons/account/views/account_invoice_view.xml

@@ -321,7 +321,7 @@
                                     <field domain="[('company_id', '=', company_id), ('internal_type', '=', 'payable'), ('deprecated', '=', False)]"
                                 name="account_id" groups="account.group_account_manager"/>
                                     <field name="name" invisible="1"/>
-                                    <field name="payment_term_id" options="{'no_create': True}" attrs="{'invisible': [('payment_term_id','=',False)]}"/>
+                                    <field name="payment_term_id" options="{'no_create': True}"/>
                                 </group>
                                 <group>
                                     <field name="fiscal_position_id" options="{'no_create': True}" placeholder="Auto-detect"/>

addons/purchase/models/account_invoice.py

@@ -75,6 +75,8 @@ class AccountInvoice(models.Model):
             new_lines += new_line
 
         self.invoice_line_ids += new_lines
+        self.payment_term_id = self.purchase_id.payment_term_id
+        self.env.context = dict(self.env.context, from_purchase_order_change=True)
         self.purchase_id = False
         return {}
 
@@ -92,7 +94,10 @@ class AccountInvoice(models.Model):
 
     @api.onchange('partner_id', 'company_id')
     def _onchange_partner_id(self):
+        payment_term_id = self.env.context.get('from_purchase_order_change') and self.payment_term_id or False
         res = super(AccountInvoice, self)._onchange_partner_id()
+        if payment_term_id:
+            self.payment_term_id = payment_term_id
         if not self.env.context.get('default_journal_id') and self.partner_id and self.currency_id and\
                 self.type in ['in_invoice', 'in_refund'] and\
                 self.currency_id != self.partner_id.property_purchase_currency_id: