You can edit your site's .htaccess file or settings.php file to do so.
This way there will not be any conflict between your site and your
drush configuration. And in case you changed your server's php version
in the future, you don't have to re-do the same changes to the new 
php version's php.ini file.
  
Via .htaccess:(put these two lines inside .htaccess file)
php_value mbstring.http_input pass
php_value mbstring.http_output pass

Via settings.php
ini_set('mbstring.http_input', 'pass');
ini_set('mbstring.http_output', 'pass');