Fix charset_types as mime.types is updated

Moved from h5bp/server-configs#134.

__Comments:__ 

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@tszming:

> The default Nginx `charset_types` [1] is 

>    `text/html text/xml text/plain text/vnd.wap.wml application/x-javascript application/rss+xml`

> Since the `mime.types` is updated and `application/javascript application/json` are now being used instead of `application/x-javascript`, if the `charset_types` is not updated, then `charset` directive will not work as expected.

> Besides, I have added `text/css` for better handling of non-ASCII in css file. [3]

> [1] http://wiki.nginx.org/HttpCharsetModule#charset_types
[2] http://forum.nginx.org/read.php?2,52360,52370#msg-52370
[3] http://trac.nginx.org/nginx/ticket/312

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@eric-brechemier:

> Agreed. This is similar to the `gzip_types` directive used in `nginx.conf`.

> The MIME type `application/xml` associated with `rss` files in `mime.types` configuration file should also be included in addition to the MIME type `application/rss+xml`.