Don't render attr if current value is same as new value

[raytiley]

This fixes #10449 at least in my app. Sorry I don't have tests yet, I can try and circle back to that later.

[stefanpenner]

the scenario makes sense, but we would need tests. It is also possible this should be handled elsewhere, maybe @tomdale or @mmun or @krisselden have further suggestions.

[mixonic]

We discussed this in IRC. I'd like to ship an alternative implementation where we inform the morph or view of the current DOM value. I'd like to avoid making value a special case at this level, and avoid reading attribute values from DOM at render in general.

The proposal is something like morph.forceActualValue(newValue) at _elementValueDidChange

[rwjblue]

We discussed this in the HTMLBars meeting today, and decided to go forward with this patch until we can fix it with a better API as @mixonic suggested.

I rebased, and added a test case that previously failed to hopefully help prevent regressions in the future.