OSRA273 fix duplicate osra_num on import

OSRA273 fix duplicate osra_num on import
Did you determine experimentally that this had to be changed? Logically, I don't think it should have to be. Validations are run before any save to the db - initially on create and subsequently on every edit. `before_save` calls the `generate_osra_num` method every time the record is saved to the db while `before_create` calls it only when the record is persisted for the first time. In fact, on the initial save, [`before_save` fires before `before_create`](http://edgeguides.rubyonrails.org/active_record_callbacks.html#creating-an-object), although this difference is inconsequential in practical terms.