Addition filter: set last_in_chain flag when clearing last_buf.

When the last_buf flag is cleared for add_after_body to append more data from a
subrequest, other filters may still have buffered data, which should be flushed
at this point.  For example, the sub_filter may have a partial match buffered,
which will only be flushed after the subrequest is done, ending up with
interleaved data in output.

Setting last_in_chain instead of last_buf flushes the data and fixes the order
of output buffers.

src/http/modules/ngx_http_addition_filter_module.c

@@ -171,6 +171,7 @@ ngx_http_addition_body_filter(ngx_http_request_t *r, ngx_chain_t *in)
     for (cl = in; cl; cl = cl->next) {
         if (cl->buf->last_buf) {
             cl->buf->last_buf = 0;
+            cl->buf->last_in_chain = 1;
             cl->buf->sync = 1;
             last = 1;
         }