Threadsafe headers

Make ShopifyAPI compatible with threadsafe fork of ActiveResource.

There's enough demand for this (it's used in at least two Shopify apps, and there's [an active forum thread about it](https://ecommerce.shopify.com/c/shopify-apis-and-technology/t/recommended-library-for-multithreaded-ruby-environment-158053)) that I think it just makes sense to conditionally include it in core.