Fix error when removing attributes from tree that contains comments

lib/readability/helper.ex

@@ -29,6 +29,7 @@ defmodule Readability.Helper do
   """
   @spec remove_attrs(html_tree, String.t() | [String.t()] | Regex.t()) :: html_tree
   def remove_attrs(content, _) when is_binary(content), do: content
+  def remove_attrs({:comment, _} = comment, _), do: comment
   def remove_attrs([], _), do: []
 
   def remove_attrs([h | t], t_attrs) do

test/readability/helper_test.exs

@@ -57,7 +57,6 @@ defmodule Readability.HelperTest do
     assert result == expected
   end
 
-
   test "inner text length", %{html_tree: html_tree} do
     result = html_tree |> Helper.text_length()
     assert result == 5
@@ -93,4 +92,10 @@ defmodule Readability.HelperTest do
     assert result_with_scheme =~ foo_url
     assert result_with_scheme =~ bar_url_https
   end
+
+  test "remove attrs with comments" do
+    tree = Floki.parse("<div class=\"foo\">hello <span><!-- world --></span></div>")
+    expected = Floki.parse("<div>hello <span><!-- world --></span></div>")
+    assert expected == Helper.remove_attrs(tree, ~w[class])
+  end
 end