headers_render: three duplicated headers in every response

headers_render: three duplicated headers in every response

Http header names are case insensitive, php array keys are not. The
transports set "Content-type" with a lowercase t (transports.phh:568 and
:618, index.php:1025 and :1199) while headers_render() checked for
"Content-Type": the check never saw it and added the default anyway.

Same dynamic for Expires and Cache-Control, which force_no_cache() sets and
headers_render added again without checking at all.

Every response therefore went out with:
  Content-Type: text/html         +  Content-type: text/html; charset="utf-8"
  Expires: -1                     +  Expires: Mon, 26 Jul 1997 05:00:00 GMT
  Cache-Control: no-cache         +  Cache-Control: no-cache, must-revalidate

With apache this went unnoticed: the daemon wrote the bytes straight to the
client and the browser applied the last value. Behind a reverse proxy the
response is parsed instead, the first value wins and the second is dropped:
the charset was lost, and in transports.phh:568 an application/xml was
replaced by text/html.

Fixed at the root rather than in the five calling places: headers_render
builds a map of the keys normalised to lowercase and uses it for every
check, so the defect does not come back if somebody writes "Content-type"
again tomorrow.

Found by putting nginx in front of the daemon: it reported "upstream sent
duplicate header line" 123 times. After the fix: zero.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014M1jiEq9cHdE5SE5j6vFuE