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.


git-svn-id: svn://svn.nginx.org/nginx/trunk@5175 73f98a42-aea0-e011-b76d-00259023448c