Convert_units() was validating categories on raw input like "kg" or "ft"
instead of the normalized unit name, so aliases got checked against a
category list that only has canonical names in it. Any alias-based
conversion that should've worked just raised ValidationError instead.
Fixed by normalizing both units before the category check runs.
Also added foot/yard/mile/gallon to the alias map - they already had
conversion factors but weren't mapped to their canonical names, so they'd
still have failed even after the above fix.
Turned out there was a second bug hiding behind the first one: the category
check defaults both sides to None, and None == None is True, so two aliases
from different categories that neither resolved to a real category would
silently pass instead of raising. kg -> ft would just return a number
instead of erroring. Normalizing first fixes this too, since aliases now
resolve to their actual categories and the mismatch gets caught.
Added a regression test locking that second one down - kg->ft and gal->lb
now raise ValidationError instead of silently converting.
Fixes#931.
hari