Mod_proxy: response sequence

Hi.

Just a short introduction. I am new to the Apache Community, but interested
in fixing bugs. Right now I am concerned with Bug #19439
(http://issues.apache.org/bugzilla/show_bug.cgi?id=19439). This bug deals
with the problem which occurs when an response has multiple
Via-Header-Fields. I've tracked it down to the function call
apr_table_compress(r->headers_in, APR_OVERLAP_TABLES_MERGE) which takes
place on incoming requests, but not when a response returns.
But I didn't get deep enough into the entire sequence of processing
responses, so I don't understand why a response is not processed similar to
a request.

Would be glad about an explanation, Robert