Fix screenshot helper to provide correct file name

We only want the file name to include the word `failures` if it failed,
not any time the user wants to take a screenshot during a test run.

actionpack/lib/action_dispatch/system_testing/test_helpers/screenshot_helper.rb

@@ -17,7 +17,7 @@ def take_screenshot
         # Takes a screenshot of the current page in the browser if the test
         # failed.
         #
-        # +take_screenshot+ is included in <tt>system_test_helper.rb</tt> that is
+        # +take_failed_screenshot+ is included in <tt>system_test_helper.rb</tt> that is
         # generated with the application. To take screenshots when a test fails
         # add +take_failed_screenshot+ to the teardown block before clearing
         # sessions.
@@ -26,8 +26,12 @@ def take_failed_screenshot
         end
 
         private
+          def image_name
+            passed? ? method_name : "failures_#{method_name}"
+          end
+
           def image_path
-            "tmp/screenshots/failures_#{method_name}.png"
+            "tmp/screenshots/#{image_name}.png"
           end
 
           def save_image

actionpack/test/dispatch/system_testing/screenshot_helper_test.rb

@@ -5,6 +5,14 @@ class ScreenshotHelperTest < ActiveSupport::TestCase
   test "image path is saved in tmp directory" do
     new_test = ActionDispatch::SystemTestCase.new("x")
 
-    assert_equal "tmp/screenshots/failures_x.png", new_test.send(:image_path)
+    assert_equal "tmp/screenshots/x.png", new_test.send(:image_path)
+  end
+
+  test "image path includes failures text if test did not pass" do
+    new_test = ActionDispatch::SystemTestCase.new("x")
+
+    new_test.stub :passed?, false do
+      assert_equal "tmp/screenshots/failures_x.png", new_test.send(:image_path)
+    end
   end
 end