Expect CORS preflight responses to not have a content type

https://github.com/cyu/rack-cors/pull/180

test/integration/cors_test.rb

@@ -9,7 +9,7 @@ class CORSTest < ActionDispatch::IntegrationTest
 
     assert_response :success
     assert_equal "*", response.headers["Access-Control-Allow-Origin"]
-    assert_equal "text/plain", response.content_type
+    assert_nil response.content_type
     assert_equal "", response.body
   end