Improving test coverage for rag

This is part of the [test coverage epic](https://www.pivotaltracker.com/epic/show/1034778), in particular for increasing [test coverage on the feature grader](https://www.pivotaltracker.com/story/show/62083218), including changes to these specs:

  - grade 3 specs
  - grader spec
  - auto_grader specs
  - feature_grader specs