Check that HTTP repeated headers are actually working

I have the strong suspicion that repeated headers only work if they are send/received as comma-separated lists, not as actual repeated headers.

The issue names no files or tests; start by locating the HTTP header handling and checking how repeated headers are parsed and emitted. Compare actual repeated headers with comma-separated values, then document the observed behavior and add coverage if the project’s existing tests support it.

Written by the indexing model from the issue text.