xml builder: properly escape quotes

lib/xml_builder.ex

@@ -35,6 +35,7 @@ def to_doc(content), do: ~s(<?xml version="1.0" encoding="UTF-8"?>) <> to_xml(co
   defp make_open_tag(tag, attributes) do
     attributes_string =
       for {attribute, value} <- attributes do
+        value = String.replace(value, "\"", "&quot;")
         "#{attribute}=\"#{value}\""
       end
       |> Enum.join(" ")