Pass in headers to restclient

connection.rb was ignoring any headers that were passed in, making things like using the "If-Modified-Since" header essentially impossible.

This PR ensures everything relevant gets passed through to RestClient::Resource.

It works in our project, but I haven't added any tests for a few reasons:

1. I don't know if there's any chance of this getting merged anyway,
2. There really aren't many other tests in the project.

Any interest in this PR bigcommerce team?