Request body: only read body in main request (ticket #330).

Before 1.3.9 an attempt to read body in a subrequest only caused problems
if body wasn't already read or discarded in a main request.  Starting with
1.3.9 it might also cause problems if body was discarded by a main request
before subrequest start.

Fix is to just ignore attempts to read request body in a subrequest, which
looks like right thing to do anyway.
diff --git a/src/http/ngx_http_request_body.c b/src/http/ngx_http_request_body.c
index 4e7b302..2c61231 100644
--- a/src/http/ngx_http_request_body.c
+++ b/src/http/ngx_http_request_body.c
@@ -49,7 +49,7 @@
     }
 #endif
 
-    if (r->request_body || r->discard_body) {
+    if (r != r->main || r->request_body || r->discard_body) {
         post_handler(r);
         return NGX_OK;
     }