set JSON_UNESCAPED_UNICODE for default json_encode option

According to [PSR-1](http://www.php-fig.org/psr/psr-1/), php code file must only use utf-8. Since the default encoding of JSON string is utf-8 too, it's time to set the `JSON_UNESCAPED_UNICODE` as the default option for json_encode method, which is a great convenient for developers come from East Asia.