Setting same property value still sets isDirty (Regression)

This was marked as fixed in #221. In fact, I confirmed this to be fixed for @NickFranceschina, but it appears to have since regressed back to broken. Here is a pull request with a test against this issue. 

My initial thought was to check in `dirtyRecordsForAttributeChange` against the `data.attribute[attributeName]` value, though I'm not confident about this. If someone can suggest a proper fix, I'd be happy to pursue it.