Backport ActiveResource patch to inherit headers from parent classes.

See rails commit c5bf53822f5887c9b235ceef388b1c8a977bcc88.

lib/active_resource/base_ext.rb

@@ -0,0 +1,14 @@
+module ActiveResource
+  class Base
+    # Backported from ActiveResource master branch
+    def self.headers
+      @headers ||= {}
+
+      if superclass != Object && superclass.headers
+        @headers = superclass.headers.merge(@headers)
+      else
+        @headers
+      end
+    end
+  end
+end

lib/shopify_api.rb

@@ -10,6 +10,7 @@ require 'shopify_api/limits'
 require 'shopify_api/json_format'
 require 'active_resource/json_errors'
 require 'active_resource/disable_prefix_check'
+require 'active_resource/base_ext'
 
 module ShopifyAPI
   include Limits

lib/shopify_api/resources/base.rb

@@ -4,11 +4,6 @@ module ShopifyAPI
   class Base < ActiveResource::Base
     extend Countable
     self.include_root_in_json = false
-
-    def self.inherited(klass)
-      super
-      klass.headers['User-Agent'] = Base.headers['User-Agent']
-    end
     self.headers['User-Agent'] = ["ShopifyAPI/#{ShopifyAPI::VERSION}",
                                   "ActiveResource/#{ActiveResource::VERSION::STRING}",
                                   "Ruby/#{RUBY_VERSION}"].join(' ')