I've added those you suggested since you are correct this needs to be processed. As for the not modified code I found the following: http://www.feedthebot.com/ifmodified.html. Indicating that you would need to specify a specific header making it an optional get. Or in other words only get the data if it has changed since the provided time-stamp. If this is correct the basically the expected behavior should be to return nothing since nothing has changed.

Having scanned the remaining HTTP Status codes it seems those 3 should cover the load.