mod_xml2enc: Fix forwarding of error metadata/responses. PR 62180.

All meta buckets are now aggregated (besides FLUSH) and forwarded down
the chain, and the output filter bails out on EOS.

Proposed by: Micha Lenk <micha lenk.info>


mod_xml2enc: follow up to r1829038.

If any, pass pending meta bucket down the chain before leaving.


mod_xml2enc: follow up to r1829038 and r1829039.

Use below pending_data logic for EOS bucket.
This closes #48


Submitted by: ylavic
Reviewed by: ylavic, jim, rpluem


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1834104 13f79535-47bb-0310-9956-ffa450edef68