Ensure collection add-on comments get saved properly

As mentioned in [Bug 961906](https://bugzilla.mozilla.org/show_bug.cgi?id=961906), any comments added to add-ons within collections were not being persisted. This should fix the issue by using slightly different syntax.

I believe the original problem stemmed from the fact that every time `c[0]` is referenced we actually get a new instance of the object.