Add support for img tags

lib/outlook/internal_tree/html.ex

@@ -5,6 +5,8 @@ defmodule Outlook.InternalTree.Html do
 
   @desirable_atts %{"a" => [:href], "img" => [:src]}
 
+  @void_elements ~w(img br hr)
+
   def strip_attributes([%InternalNode{type: :element} = node | rest]) do
     d_atts = Map.get(@desirable_atts, node.name, [])
     atts = Map.reject(node.attributes, fn {k,_} -> k not in d_atts end)
@@ -22,6 +24,11 @@ defmodule Outlook.InternalTree.Html do
   def strip_attributes([]), do: []
 
 
+  def to_html([%{type: :element} = node | rest]) when node.name in @void_elements do
+    attr_string = Enum.map_join(node.attributes, "", fn {k,v} -> " #{k}=\"#{v}\"" end)
+    "<#{node.name}#{attr_string}>" <> to_html(rest)
+  end
+
   def to_html([%{type: :element} = node | rest]) do
     attr_string = Enum.map_join(node.attributes, "", fn {k,v} -> " #{k}=\"#{v}\"" end)
     "<#{node.name}#{attr_string}>" <>