Fix handling of BigDecimal values after money gem upgrade

After the Money gem upgrade from 3.6.1 to 6.1.1, the spec assets changed to show all values as 100 times smaller than they were previously.  Upgrading Payday in my project has the same effect.

This is because the core extension `#to_money` in 3.6.1 does not behave in quite the same way as `Money.new` in 6.1.1.  These changes restore the old behavior using the implementation from the [Monetize](https://github.com/RubyMoney/monetize/blob/master/lib/monetize.rb#L55) gem.