Fix body with request_body_in_single_buf.
Fix body with request_body_in_single_buf.

If there were preread data and request body was big enough first part
of the request body was duplicated.

See report here:
http://mailman.nginx.org/pipermail/nginx/2011-July/027756.html

hy0kl/nginx,firebase/nginx,firebase/nginx,firebase/nginx,hy0kl/nginx,firebase/nginx,hy0kl/nginx