Fix Product.price_range method for variants with different prices.
Fix Product.price_range method for variants with different prices.

Fixes #12.

ifnull/shopify_python_api,SmileyJames/shopify_python_api,gavinballard/shopify_python_api,asiviero/shopify_python_api,varesa/shopify_python_api,metric-collective/shopify_python_api,Shopify/shopify_python_api