Fix headers not being parsed due to case differences

Commit 8cd310be broke case-insensitive header parsing since it went
from using GMime's header class to using a Gee.Map for header response
lookup. This patch makes the map use case-insensitive keys.
4 jobs for cherry-pick-737bdba1 in 31 minutes and 58 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #486232
ubuntu

00:13:01

 
  Test
passed #486233
flatpak

00:18:25

 
  Deploy
passed #486234
review

00:00:32

failed #486235
allowed to fail manual
stop_review

 
Name Stage Failure
failed
stop_review Deploy There has been a missing dependency failure
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