Validate input arguments.

[Krolken]

It is possible to supply a model with wrong input arguments and this might lead to unexpected errors.

for exemple

invoice = CustomerInvoiceDraft(customer_id=customer.id,
                                       invoe_customer_name='test',
                                       invoice_postal_code='25459',
                                       invoice_city='Helsingborg',
                                       customer_is_private_person=False)

We should validate that all input arguments to creating of an object acctually exists as a field on the model.