Discard request body when redirecting to a URL via error_page.

Reported by Bert JW Regeer and Francisco Oca Gonzalez.

src/http/ngx_http_special_response.c

diff --git a/src/http/ngx_http_special_response.c b/src/http/ngx_http_special_response.c
index 4ffb2cc..76e6705 100644
--- a/src/http/ngx_http_special_response.c
+++ b/src/http/ngx_http_special_response.c
@@ -623,6 +623,12 @@
         return ngx_http_named_location(r, &uri);
     }
 
+    r->expect_tested = 1;
+
+    if (ngx_http_discard_request_body(r) != NGX_OK) {
+        r->keepalive = 0;
+    }
+
     location = ngx_list_push(&r->headers_out.headers);
 
     if (location == NULL) {