Do operation base on charset

[mutexd]

Original feature convert all response regardless the charset indicated in Content-Type. We should be able to convert it only when charset matched.
For example, gb2312->utf-8, this module only convert charset when response charset is gb2312. And it need to modify header to utf-8 (just like ngx_charset_filter_module).

This feature should be able to turn-on and off by directive.