Fix for "return 202" not discarding body.

Fix for "return 202" not discarding body.

Big POST (not fully preread) to a

    location / {
        return 202;
    }

resulted in incorrect behaviour due to "return" code path not calling
ngx_http_discard_request_body().  The same applies to all "return" used
with 2xx/3xx codes except 201 and 204, and to all "return ... text" uses.

Fix is to add ngx_http_discard_request_body() call to ngx_http_send_response()
function where it looks appropriate.  Discard body call from emtpy gif module
removed as it's now redundant.

Reported by Pyry Hakulinen, see
http://mailman.nginx.org/pipermail/nginx/2011-August/028503.html