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.