TODO removal

For each of these TODOs:

- If there's an active bug tracking the work, and the TODO provides
  something of value, I left it and standardized the formatting. I also
  added comments to the bug.

- If the comment provided no value (implement feature X when we do feature
  X), I deleted it with impunity.

- If the comment was stale (won't fix or just out of date), then we
  removed it uncerimoniously.

There was a single TODO that was actually actionable, so I enabled that
test.
Could you simply uncomment this line? The `BodyModelValidator` already uses this - https://github.com/aspnet/Mvc/blob/e21f15709509628cc33995bed3dd23800a2cccd5/src/Microsoft.AspNet.Mvc.ModelBinding/Validation/DefaultBodyModelValidator.cs#L40 - so we know it's reliable.