Missed a MASSIVE set of failures in the sadd spec the first time

arround. This corrals the dump call to arrays hashes and strings

lib/mock_redis/utility_methods.rb

@@ -9,12 +9,16 @@ def with_thing_at(key, assertion, empty_thing_generator)
         data_key_ref = data[key]
         ret = yield data[key]
         data[key] = data_key_ref if data[key].nil?
-        ret ? ret.dup : ret
+        primitive?(ret) ? ret.dup : ret
       ensure
         clean_up_empties_at(key)
       end
     end
 
+    def primitive?(value)
+      value.kind_of?(::Array) || value.kind_of?(::Hash) ||value.kind_of?(::String)
+    end
+
     def clean_up_empties_at(key)
       if data[key] && data[key].empty?
         del(key)