ensure coerce only applies to multiplication

I agree that we should either explicitly request for `#to_money` conversions or imply that the numeric is in the base currency. One or the other. But it should be consistent and that means we shouldn't allow any operations between `Numeric` and `Money`, not even `Money.new(2, 'USD') * 2` which is currently allowed.

Personally I'd go the other way around (coerce values) but that solution (require explicit `#to_money` conversion) is acceptable as well.