Fixed keg volume calculation on non-charge liquids (such as water).

Previously charge liquids (such as wine) would calculate correctly
(volume_per_item * charges / charges_per_item) but non-charge liquids
would calculate as (volume_per_item * charges * charges).

A keg containing only the contents of a plastic bottle (capacity 2)
completely filled with fruit wine (volume 2, charges_per_item 7) would
correctly have a usage of 4, while one containing only the contents of
a gallon jug (capacity 15) would correctly have a usage of 30.

Without this fix, a keg filled with a plastic bottle of water would
incorrectly have a usage of 8 (2x the correct value) and a keg filled
with a gallon jug of water would incorrectly have a usage of 450 (15x
the correct value).