[Fix] Not able to save sales order

[rohitwaghchaure]

Traceback (most recent call last):
  File "/home/frappe/benches/bench-2018-02-08/apps/frappe/frappe/desk/form/save.py", line 22, in savedocs
    doc.save()
  File "/home/frappe/benches/bench-2018-02-08/apps/frappe/frappe/model/document.py", line 256, in save
    return self._save(*args, **kwargs)
  File "/home/frappe/benches/bench-2018-02-08/apps/frappe/frappe/model/document.py", line 279, in _save
    self.insert()
  File "/home/frappe/benches/bench-2018-02-08/apps/frappe/frappe/model/document.py", line 218, in insert
    self.run_before_save_methods()
  File "/home/frappe/benches/bench-2018-02-08/apps/frappe/frappe/model/document.py", line 859, in run_before_save_methods
    self.run_method("validate")
  File "/home/frappe/benches/bench-2018-02-08/apps/frappe/frappe/model/document.py", line 755, in run_method
    out = Document.hook(fn)(self, *args, **kwargs)
  File "/home/frappe/benches/bench-2018-02-08/apps/frappe/frappe/model/document.py", line 1024, in composer
    return composed(self, method, *args, **kwargs)
  File "/home/frappe/benches/bench-2018-02-08/apps/frappe/frappe/model/document.py", line 1007, in runner
    add_to_return_value(self, fn(self, *args, **kwargs))
  File "/home/frappe/benches/bench-2018-02-08/apps/frappe/frappe/model/document.py", line 749, in <lambda>
    fn = lambda self, *args, **kwargs: getattr(self, method)(*args, **kwargs)
  File "/home/frappe/benches/bench-2018-02-08/apps/erpnext/erpnext/selling/doctype/sales_order/sales_order.py", line 30, in validate
    super(SalesOrder, self).validate()
  File "/home/frappe/benches/bench-2018-02-08/apps/erpnext/erpnext/controllers/selling_controller.py", line 39, in validate
    self.validate_selling_price()
  File "/home/frappe/benches/bench-2018-02-08/apps/erpnext/erpnext/controllers/selling_controller.py", line 159, in validate_selling_price
    if flt(it.base_rate) < flt(last_purchase_rate_in_sales_uom) and not self.is_return:
AttributeError: 'SalesOrder' object has no attribute 'is_return'