ensure coerce only applies to multiplication

In python-money, `Money.new(100, 'USD') + Money.new(100, 'EUR')` equals `Money.new(100 + Money.new(100, 'EUR').exchange_to('USD').fractional, 'USD')` as far as I understand the code.
Also, `Money.new(100, 'USD') * Money.new(100, 'USD')` raises an error.

Both makes perfectly sense. That's adding coercion consistently to the rules I outlined in https://github.com/RubyMoney/money/pull/405#issuecomment-47310481

You are right. Let's stop writing prose and start writing code :-)
I'll try to find time to follow you.