mark hex with 0x

src/cryptoki.hxx

@@ -57,7 +57,7 @@ namespace cryptoki {
 
   inline std::string readable(const std::string& data) {
     if (data.find_first_not_of(VALID_CHARS)<data.size())
-      return hex(data);
+      return "0x"+hex(data);
     else
       return data;
   }