Add missing specs for List

* Fixes https://github.com/opscode/chef/issues/2245
* No TODOs to fix.
* Looks like `ResourceCollection` has some internal logic during inserts and it's not super trivial to share examples across classes. In the interest of time I think we should drop the example sharing idea.
* @danielsdeleo is it safe to get rid of json serialization in the ResourceCollection?
* Will add RELEASE_NOTES & CHANGELOG updates before merge.

/cc: @tyler-ball 
Since you're already validating this in its own test, I would say we don't need the assertion here too.

It also looks like this same code is duplicated a lot of places.  Can you DRY it up with a method?