Move template_format logic out to the request so it's alongside the 'regular' request format.

Use xhr? instead of the expensive trip through Request#accepts.

actionpack/lib/action_controller/request.rb

@@ -116,6 +116,19 @@ module ActionController
       @format = Mime::Type.lookup_by_extension(parameters[:format])
     end
 
+    def template_format
+      parameter_format = parameters[:format]
+
+      case
+      when parameter_format.blank? && !xhr?
+        :html
+      when parameter_format.blank? && xhr?
+        :js
+      else
+        parameter_format.to_sym
+      end
+    end
+
     # Returns true if the request's "X-Requested-With" header contains
     # "XMLHttpRequest". (The Prototype Javascript library sends this header with
     # every Ajax request.)

actionpack/lib/action_view/base.rb

@@ -273,17 +273,7 @@ module ActionView #:nodoc:
       return @template_format if @template_format
 
       if controller && controller.respond_to?(:request)
-        parameter_format = controller.request.parameters[:format]
-        accept_format    = controller.request.accepts.first
-
-        case
-        when parameter_format.blank? && accept_format != :js
-          @template_format = :html
-        when parameter_format.blank? && accept_format == :js
-          @template_format = :js
-        else
-          @template_format = parameter_format.to_sym
-        end
+        @template_format = controller.request.template_format
       else
         @template_format = :html
       end

actionpack/test/controller/new_render_test.rb

@@ -605,8 +605,7 @@ EOS
   end
 
   def test_render_with_default_from_accept_header
-    @request.env["HTTP_ACCEPT"] = "text/javascript"
-    get :greeting
+    xhr :get, :greeting
     assert_equal "$(\"body\").visualEffect(\"highlight\");", @response.body
   end