NOJIRA - Resolve shared examples error

Renamed models/canvas/authorization_helpers_spec.rb to models/canvas/authorization_helpers_shared.rb to resolve warning during tests.

```bash
WARNING: Shared example group 'an api endpoint' has been previously defined at:
 calcentral/spec/models/canvas/authorization_helpers_spec.rb:135
...and you are now defining it at:
calcentral/spec/models/canvas/authorization_helpers_spec.rb:135
The new definition will overwrite the original one.
```

http://jimmstout.com/2013/shared-examples-for-testing-session-links/