Allow custom asset host to be passed in asset_url

actionview/lib/action_view/helpers/asset_url_helper.rb

@@ -191,7 +191,8 @@ def compute_asset_path(source, options = {})
       # (proc or otherwise).
       def compute_asset_host(source = "", options = {})
         request = self.request if respond_to?(:request)
-        host = config.asset_host if defined? config.asset_host
+        host = options[:host]
+        host ||= config.asset_host if defined? config.asset_host
         host ||= request.base_url if request && options[:protocol] == :request
 
         if host.respond_to?(:call)

actionview/test/template/asset_tag_helper_test.rb

@@ -596,6 +596,10 @@ def test_should_current_request_host_is_always_returned_for_request
     assert_equal "gopher://www.example.com", compute_asset_host("foo", :protocol => :request)
   end
 
+  def test_should_return_custom_host_if_passed_in_options
+    assert_equal "http://custom.example.com", compute_asset_host("foo", :host => "http://custom.example.com")
+  end
+
   def test_should_ignore_relative_root_path_on_complete_url
     assert_dom_equal(%(http://www.example.com/images/xml.png), image_path("http://www.example.com/images/xml.png"))
   end
@@ -759,4 +763,15 @@ def config
     assert @module.config.asset_host
     assert_equal "http://www.example.com/foo", @module.asset_url("foo")
   end
+
+  def test_asset_url_with_custom_asset_host
+    @module.instance_eval do
+      def config
+        Struct.new(:asset_host).new("http://www.example.com")
+      end
+    end
+
+    assert @module.config.asset_host
+    assert_equal "http://custom.example.com/foo", @module.asset_url("foo", :host => "http://custom.example.com")
+  end
 end