Dragging an image into a collection that has a parent in Firefox causes a confirmation popup

[bmaddy]

To reproduce:

• use Firefox
• drag an image from the search results onto a personal collection that has a parent (the error doesn't happen on top-level collections)
• The following appears: "This web page is being redirected to a new location. Would you like to resend the form data you have typed to the new location?" and the image is not added to the collection.

Expected:

• [x] the confirmation doesn't appear
• [x] the image is added to the collection

[jcoyne]

It's because it was trying to add an image to the personal collection by posting to the CourseCollectionController.