Duktape: Use internal properties (prefixed by \xFF).

According to the documentation http://duktape.org/guide.html#internalproperties
and reccomendations https://github.com/svaarala/duktape/issues/392#issuecomment-145301192

http://duktape.org/guide.html#internalproperties

src/mod_duktape.inl

@@ -14,7 +14,7 @@
 /* Note: This is only experimental support, so any API may still change. */
 
 
-static const char *civetweb_conn_id = "civetweb_conn";
+static const char *civetweb_conn_id = "\xFF" "civetweb_conn";
 
 
 static void *mg_duk_mem_alloc(void *udata, duk_size_t size)