Add illustrative test around Money object
Add illustrative test around Money object

I couldn't understand why I was seeing validation errors when entering negative
numbers in response to Money questions. It turns out that the Money object
itself will raise an `InvalidResponse` exception if it's constructed with a
negative number.

tadast/smart-answers,aledelcueto/smart-answers,askl56/smart-answers,askl56/smart-answers,aledelcueto/smart-answers,aledelcueto/smart-answers,stwalsh/smart-answers,aledelcueto/smart-answers,alphagov/smart-answers,alphagov/smart-answers,stwalsh/smart-answers,alphagov/smart-answers,tadast/smart-answers,stwalsh/smart-answers,alphagov/smart-answers,stwalsh/smart-answers,askl56/smart-answers,tadast/smart-answers,tadast/smart-answers,askl56/smart-answers