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

src/http/ngx_http_request_body.c

diff --git a/src/http/ngx_http_request_body.c b/src/http/ngx_http_request_body.c
index 57c201b..03053c8 100644
--- a/src/http/ngx_http_request_body.c
+++ b/src/http/ngx_http_request_body.c
@@ -372,7 +372,9 @@
         }
     }
 
-    if (r->request_body_in_file_only && rb->bufs->next) {
+    if (rb->bufs->next
+        && (r->request_body_in_file_only || r->request_body_in_single_buf))
+    {
         rb->bufs = rb->bufs->next;
     }