control/controlhttp: fix header case-sensitivity

Change-Id: I49269bc969a80382997ec5c9de33c4f56d9dc787
Signed-off-by: Matthias Gabriel <matthias.gabriel@etit.tu-chemnitz.de>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f