BUG FIX: Only do string serialization of collapsed AuxCoords

This fixes a bug introduced by #847, which is encountered when attempting to collapse along a dimension coordinate with unknown units. In such a case, the coordinate value is converted into a string, which fails for dimension coordinates because dimension coordinates must be numeric.

Apologies for the spurious lines in the diff -- it's just removing extraneous whitespace, which my editor does by default on save.