Fixed aggregate_by bug for single values coordinates.

Closes #1183.
Nope, I'm always using the two bounds from the slice, even if they are the same thing.
I believe this to be consistent with the existing behaviour (when it doesn't break) - I was tempted to try making this better (and did start doing so) but it wasn't as trivial a fix. If you want me to though, I'd happily look into the prospect of only joining if there is more than one value.