ensure coerce only applies to multiplication

Wrt. coercion, I'd expect that `1 + Money.new(100)` should be ... uhm, `Money.new(101)` or `Money.new(200)`? There's a built in ambiguity that can be fixed "by convention", but I don't think it's a great solution. Forcing the user to explicitly call `#to_money` or `Money.new` is a better solution IMO.