fix edge case where sanitize gem doesn't operate on <a> node if the custom transformer runs

lib/gollum/sanitization.rb

@@ -47,27 +47,36 @@ module Gollum
       'img' => {'src'  => ['http', 'https', :relative]}
     }.freeze
 
-    # Default transformers to force @id attributes with 'wiki-' prefix
+    ADD_ATTRIBUTES = lambda do |env, node|
+      if add = env[:config][:add_attributes][node.name]
+        add.each do |key, value|
+          node[key] = value
+        end
+      end
+    end
 
+    # Default transformers to force @id attributes with 'wiki-' prefix
     TRANSFORMERS = [
       lambda do |env|
         node      = env[:node]
         return if env[:is_whitelisted] || !node.element?
         prefix = env[:config][:id_prefix]
-        %w(id name).each do |key|
+        found_attrs = %w(id name).select do |key|
           if value = node[key]
             node[key] = value.gsub(/\A(#{prefix})?/, prefix)
           end
         end
-
-        {:node_whitelist => [node]}
+        if found_attrs.size > 0
+          ADD_ATTRIBUTES.call(env, node)
+          {:node_whitelist => [node]}
+        end
       end,
       lambda do |env|
         node = env[:node]
-        return unless node['href']
+        return unless value = node['href']
         prefix = env[:config][:id_prefix]
-        node['href'] = node['href'].gsub(/\A\#(#{prefix})?/, '#'+prefix)
-
+        node['href'] = value.gsub(/\A\#(#{prefix})?/, '#'+prefix)
+        ADD_ATTRIBUTES.call(env, node)
         {:node_whitelist => [node]}
       end
     ].freeze