BUG/MINOR: mux-h1: convert the <from> argument of h1-case-adjust to lower case

BUG/MINOR: mux-h1: convert the <from> argument of h1-case-adjust to lower case

The "h1-case-adjust" global directive (and the file loaded with
"h1-case-adjust-file") takes the header name to match in lower case, as
specified in the documentation, but it was not checked. So with:

    h1-case-adjust X-Up X-Up

the rule was accepted without any warning but never matched since header
names are looked up in lower case, and "x-up" was still emitted. Let's
convert <from> to lower case before inserting it in the map, which also
makes duplicate detection work regardless of the case.

It could be backported to all supported versions.