ensure coerce only applies to multiplication

@fotos I disagree. "(4 dollars) * (4 dollars)" is "4 dollars^2" which doesn't make sense. In everyday life, I've never multiplied currencies. If "(4 dollars) * (4 dollars)" should make sense, then what should "(4 dollars) * (4 euros)" mean? And should the result be in euro or dollar? I don't like that kind of ambiguity.

*If* `4 + Money.new(x, 'USD')` should somehow make sense, then, yes, indeed, it should equal `Money.new(x + 4, 'USD')`.

I'd much prefer an explicit `#to_money` call rather than weirdness like `Money.new(100, 'EUR') + Money.new(100, 'USD')` to not raise an error.