Request body: removed error assumption (ticket #2058).

Before introduction of request body filter in 42d9beeb22db, the only
possible return code from the ngx_http_request_body_filter() call
without actual buffers was NGX_HTTP_INTERNAL_SERVER_ERROR, and
the code in ngx_http_read_client_request_body() hardcoded the only
possible error to simplify the code of initial call to set rb->rest.

This is no longer true after introduction of request body filters though,
as a request body filter might need to return other errors, such as 403.
Fix is to preserve the error code actually returned by the call
instead of assuming 500.

src/http/ngx_http_request_body.c

diff --git a/src/http/ngx_http_request_body.c b/src/http/ngx_http_request_body.c
index 07e78d7..0cae88f 100644
--- a/src/http/ngx_http_request_body.c
+++ b/src/http/ngx_http_request_body.c
@@ -137,8 +137,9 @@
     } else {
         /* set rb->rest */
 
-        if (ngx_http_request_body_filter(r, NULL) != NGX_OK) {
-            rc = NGX_HTTP_INTERNAL_SERVER_ERROR;
+        rc = ngx_http_request_body_filter(r, NULL);
+
+        if (rc != NGX_OK) {
             goto done;
         }
     }