[FIX] product: bigger ratio

- Create a new unit of measure with 'Bigger than the reference Unit of
  Measure'
- Save
- Edit and change the ratio ==> the ratio is set back to its value

opw-695507

addons/product/models/product_uom.py

@@ -61,6 +61,13 @@ class ProductUoM(models.Model):
             values['factor'] = factor_inv and (1.0 / factor_inv) or 0.0
         return super(ProductUoM, self).create(values)
 
+    @api.multi
+    def write(self, values):
+        if 'factor_inv' in values:
+            factor_inv = values.pop('factor_inv')
+            values['factor'] = factor_inv and (1.0 / factor_inv) or 0.0
+        return super(ProductUoM, self).write(values)
+
     @api.model
     def name_create(self, name):
         """ The UoM category and factor are required, so we'll have to add temporary values