Transform minFraud response values to their types

Review @manygrams @disaacs 

## Background:
In BladeRunner we are using this gem. There were only couple things that was missing from the gem and this PR adds one of them. (See the other one at #1).

In Shopify we do some kind of [parsing](https://github.com/Shopify/shopify/blob/75f6d3d8c9e892f0723ca415e8208980ef18da28/lib/min_fraud.rb#L20-L59) on the response we receive from minFraud. Even though this app does a similar kind of parsing, it doesn't do the [type conversion](https://github.com/Shopify/shopify/blob/75f6d3d8c9e892f0723ca415e8208980ef18da28/lib/min_fraud.rb#L38-L54).

This PR adds the type conversion to the gem so we don't have to worry about it in BladeRunner.

You should update the docstring above