Add Nori::StringWithAttributes#as_json

Fixes savonrb#99

Author: HarlemSquirrel

Diff for: lib/nori/string_with_attributes.rb

@@ -3,5 +3,9 @@ class StringWithAttributes < String
 
     attr_accessor :attributes
 
+    def as_json
+      to_s
+    end
+
   end
 end