Fix app/controllers/instructor/sections_controller.rb to work with AJAX

Hey Nan -

Here are the changes you need to make to fix your app.  Descriptions inline.

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As a side note, this is a GitHub pull request. This is how I can show you code that I think you should add to your project. It allows you to look at the code, you'll see the green code is code that's added and the red is code that's deleted. The "Files changed" tab will show you the code.

"Commits" will show you a listing of the individual commits I'm trying to send you. And this tab, the conversation tab you can use to communicate about the change.

If you want to suck in all this code into your project you can by pressing the green merge button. After you press the merge button, you'll have to update your web dev environment to have the latest updates from GitHub and to do that you can run this command:

```
git pull origin master
```

Hope this helps.

Aloha,
Ken

make sure to render some value otherwise you'll get an error message back from your AJAX method.  It doesn't matter what response you get so long as it's successful.