Patch for review - Reset dirty attributes #1954

Hi,

I've attached an updated patch to lighthouse ticket 1954 which
implements the resetting of attributes on a model to their original
values. The ticket was stale at the time I updated it and remained
stale afterwards and so I'm not sure how many people would have seen
it.

I'd appreciate it if someone could 'unstale' the ticket if that makes
sense and appreciate it even more if others could review the patch.

Paul