Fix casing of Content-Type header

Fix casing of Content-Type header

It turns out, despite the HTTP spec declaring that header field names
should be case-insensitive, some clients treat them wrongly, and
consider "Content-type" to not match "Content-Type".

CyberDuck was one of those clients, now fixed upstream in
https://trac.cyberduck.io/ticket/8999

To reduce future occurances of this bug, fix the casing of the
Content-Type header, to strictly comply with the HTTP specification (be
strict about what you send, and generous about what you receive).

Fixes: #12939
Backport: infernalis, hammer, firefly
Signed-off-by: Robin H. Johnson <robin.johnson@dreamhost.com>