Ruby 2.0.0 support

The main fix here is `coerce`, see the commit for a more detailed description (tl;dr `method_missing` is evil).

This fixes [this line in shipment packer](https://github.com/Shopify/active_shipping/blob/master/lib/active_shipping/shipping/shipment_packer.rb#L26) in 2.0.0.

I also made the quantified tests pass on 2.0.0.

Please review @csaunders