LMS Rails 5.1 Prework - Misc Rails 5.1 spec failures

[brendanshean]

WHAT

1. Fix calls to symbolize_keys on ActionController::Parameters object
2. Rewrite xhr controller spec requests
3. Remove duplicate has_many declarations
4. Update render :text calls due to deprecation

WHY

These items are all spec failures when upgrading to Rails 5.1. Fixing them before the actual upgrade makes the eventual upgrade diff more manageable.

HOW

1. Add to_h before calling symbolize_keys
2. change verb from xhr :get :index to get :index, xhr: true
3. Remove duplicate
4. Update render :text to render :plain

Screenshots

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Notion Card Links

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PR Checklist	Your Answer
Have you added and/or updated tests?	YES
Have you deployed to Staging?	YES
Self-Review: Have you done an initial self-review of the code below on Github?	YES
Design Review: If applicable, have you compared the coded design to the mockups?	N/A