header and body filter implementations


## config

```nginx
server {
        listen       8000;
        server_name  localhost;
        root html;

        location / {
            mruby_header_filter_code  "
                r = Nginx::Request.new()
                r.content_type = 'image/png'
            ";
            mruby_body_filter_code "
                '[' + ARGV[0] + ']'
            ";
        }
}
```

## index.html

```html
<html>
<head>
<title>Welcome to nginx!</title>
</head>
<body bgcolor="white" text="black">
<center><h1>Welcome to nginx!!!</h1></center>
</body>
</html>
```

## example

```zsh
$ curl "http://localhost:8000/index.html"
[<html>
<head>
<title>Welcome to nginx!</title>
</head>
<body bgcolor="white" text="black">
<center><h1>Welcome to nginx!!!</h1></center>
</body>
</html>
]
$  curl -I "http://localhost:8000/index.html"
HTTP/1.1 200 OK
Server: nginx/1.5.3
Date: Wed, 24 Jul 2013 16:59:32 GMT
Content-Type: image/png
Content-Length: 155
Last-Modified: Wed, 21 Mar 2012 05:26:56 GMT
Connection: keep-alive
ETag: "4f6966a0-99"
Accept-Ranges: bytes

$
```