literal ipv6 addresses should be enclosed by brackets

Nginx will truncate the ipv6 address after the first `:`. With this patch this issue is repaired.
Before:
`2a00 2a00:1630:13:1::b0 - - [27/Nov/2013:11:35:17 +0100]  "GET / HTTP/1.0" 200 233 "-" "KeepAliveClient" - - 141906 0.000`

After
`[2a00:1630:13:1::b2] 2a00:1630:13:1::b1 - - [27/Nov/2013:11:39:01 +0100]  "GET / HTTP/1.0" 200 233 "-" "KeepAliveClient" - - 142290 0.00 1`