add test

Author: joshua-bouv

test/phoenix_html/safe_test.exs

@@ -62,6 +62,11 @@ defmodule Phoenix.HTML.SafeTest do
     assert Safe.to_iodata(datetime) == "2000-01-01T12:13:14+00:30"
   end
 
+  test "impl for Duration" do
+    duration = Duration.new!(month: 1)
+    assert Safe.to_iodata(duration) == "P1M"
+  end
+
   test "impl for URI" do
     uri = %URI{scheme: "http", host: "www.example.org", path: "/foo", query: "secret=<a&b>"}