Escape slashes in log output for JSON format

[kay]

I've done the patch for nginx 1.6.2

Now if you use this config:
access_log  logs/access.log  main json;

Additional backslash for backslash should appear in log. And it will not
break JSON format.

Can someone review my changes?
https://github.com/kayrus/nginx/commit/682f3684ea331e089aa124fe7fce8409e13c1870

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,255071,255142#msg-255142