O294 sponsorship end date validation

https://osraav.atlassian.net/browse/OSRA-294
This is connected to:  https://osraav.atlassian.net/browse/OSRA-298

Sponsorship model validatation for end_date
- can't be before start date
- can't be blank

plus rspec and cucumber tests

Andrey and me worked on this.
I guess I am thinking too slowly :-) It is an http PUT test (line 21) and checks that the controller will send correct instance variables to the view. I am not sure whether it's terribly useful, but it's a valid question to ask of a controller, no?