BUG/MEDIUM: mux-h1: Always adjust case for all outgoing headers as expected

In HTX, all header names are converted to lowercase. However, some legacy H1
applications are still sensitive to the header names case.  For this
purpose, it is possible to provide a map to automatically adjust the case of
header names. While it is performed for most responses, it is not true for
the low-level errors triggered during requests parsing. It the same ways,
the case of "Sec-Websocket-Key" a "Sec-Websocket-Accept" headers were
adjusted as expected.

To fix this issue the h1-htx API was slightly changed. Now the map used to
adjust the case of header names, if any, is passed to the function
responsible to format the headers. In the H1 multiplexer, the map is first
retrieved then passed as argument to h1_format_htx_hdr() and
h1_format_htx_msg() functions. A NULL pointer is passed if no rewrite must
be performed.

In the H1 multiplexer, h1_adjust_case_outgoing_hdr() was replaced by
h1_get_hdrs_map().

All other calls to h1_format_htx_hdr() were adapted to use a NULL pointer
(httpclient, http-fetch).

This patch relies on "REORG: h1-htx: Move h1 headers map in h1-htx". Both
commits should be backported to all supported versions.

This should fix the issue #3448.