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.