use product.round_price() to round unit_price of invoice-line

mixin.py

@@ -4,13 +4,13 @@
 # full copyright notices and license terms.
 
 
-from decimal import Decimal, ROUND_HALF_EVEN
+from decimal import Decimal
 import html
 from trytond.exceptions import UserError
 from trytond.i18n import gettext
 from trytond.tools import cached_property
 from trytond.pool import Pool
-from trytond.modules.product import price_digits
+from trytond.modules.product import round_price
 
 
 class EdocumentMixin(object):
@@ -193,10 +193,8 @@ class EdocumentMixin(object):
         Returns:
             Decimal: rounded value
         """
-        if isinstance(value, Decimal):
-            return value.quantize(
-                Decimal(str(1/10 ** price_digits[1])),
-                ROUND_HALF_EVEN)
+        if value is not None:
+            return round_price(value)
         return value
 
     def quote_text(self, text):