Fix merging num/str validations that are equal

[rkusa]

While trying to use progenitor with the DHL Express API, I received the following unimplemented panic: this is fairly fussy and I don't want to do it.

I tracked the panic down to merge_so_number in this repo.

The validation rules it tried to merge where the same, which is why I were able to fix the panic by adding another case that handles if both a and b are defined but simply the same.

[ahl]

Thanks for this step forward; if you decide this isn't too fussy to do, a contribution to merge the constraints would be much appreciated.