Merge commit from fork

* add guard clause to check for node attributes

* check for encoding attribute specifically

* change spec

lib/sanitize_ext/sanitize_config.rb

@@ -83,8 +83,10 @@ class Sanitize
       # next, we find the plain-text description
       is_annotation_with_encoding = lambda do |encoding, node|
         return false unless node.name == 'annotation'
+        encoding_attr = node.attributes['encoding']
+        return false if encoding_attr.nil?
 
-        node.attributes['encoding'].value == encoding
+        encoding_attr.value == encoding
       end
 
       annotation = semantics.children.find(&is_annotation_with_encoding.curry['application/x-tex'])