Hacking ActiveResource::Connection for Shopify API in order to read HTTP response header 'http_x_shopify_api_call_limit'. Simply require this code after your gems have been loaded.

ShopifyAPICallLimit.rb

class ActiveResource::Connection
  # HACK 1:  Add an attr_reader for response
  attr_reader :response
  
  def request(method, path, *arguments)
    result = ActiveSupport::Notifications.instrument("request.active_resource") do |payload|
      payload[:method]      = method
      payload[:request_uri] = "#{site.scheme}://#{site.host}:#{site.port}#{path}"
      payload[:result]      = http.send(method, path, *arguments)
    end  
    # HACK 2:  Save response to instance var @response  
    @response = handle_response(result)
  rescue Timeout::Error => e
    raise TimeoutError.new(e.message)
  rescue OpenSSL::SSL::SSLError => e
    raise SSLError.new(e.message)
  end  
end

module ShopifyAPI
  ##
  # returns a Hash containing current API call count and limit.  If no API call has yet been executed, a call to 
  # ShopifyAPI::Shop.current will be executed in order to retrieve a first response.
  # @return {:count => Integer, :limit => Integer}
  #
  def self.call_limit
    unless ActiveResource::Base.connection.response
      Shop.current
    end    
    global = ActiveResource::Base.connection.response['http_x_shopify_api_call_limit'].split('/')
    local = ActiveResource::Base.connection.response['http_x_shopify_shop_api_call_limit'].split('/')
    {:global_count => global[0], :global_limit => global[1], :local_count => local[0], :local_limit => local[1]}
  end
end