[FIX] purchase: Only use purchase UOM on vendor bill

It was the purpose of this commit fea667a243
However it uses purchase uom in any cases and not
only vendor bill.

This commit correct it by using it only if a purchase
order is linked to the invoice.

addons/account/models/account_invoice.py

@@ -1316,9 +1316,6 @@ class AccountInvoiceLine(models.Model):
                 self.price_unit = 0.0
             domain['uom_id'] = []
         else:
-            # Use the purchase uom by default
-            self.uom_id = self.product_id.uom_po_id
-
             if part.lang:
                 product = self.product_id.with_context(lang=part.lang)
             else:

addons/purchase/models/account_invoice.py

@@ -59,6 +59,13 @@ class AccountInvoice(models.Model):
             data['account_id'] = account.id
         return data
 
+    def _onchange_product_id(self):
+        domain = super(AccountInvoice, self)._onchange_product_id()
+        if self.purchase_id:
+            # Use the purchase uom by default
+            self.uom_id = self.product_id.uom_po_id
+        return domain
+
     # Load all unsold PO lines
     @api.onchange('purchase_id')
     def purchase_order_change(self):