SPDY: fixed request hang with the auth request module.

We should just call post_handler() when subrequest wants to read body, like
it happens for HTTP since rev. f458156fd46a.  An attempt to init request body
for subrequests results in hang if the body was not already read.

src/http/ngx_http_request_body.c

@@ -43,7 +43,7 @@ ngx_http_read_client_request_body(ngx_http_request_t *r,
     r->main->count++;
 
 #if (NGX_HTTP_SPDY)
-    if (r->spdy_stream) {
+    if (r->spdy_stream && r == r->main) {
         rc = ngx_http_spdy_read_request_body(r, post_handler);
         goto done;
     }