Fix: Workaround for broken HTTP implementations

Old Icecast versions (lower than 2.4.0), all current icecast-kh versions
and derivates of that like the AzuraCast kh-ac fork do not treat the
HTTP headers in the response as case-insensitive even though this is
required by the HTTP RFC.
Go by default canonicalizes the Headers, to prevent that and actually
write our case-sensitive keys we need to access the map directly.
This adds a helper that does so, tests for the helper and uses the
helper in the relevant places in yp.go.
3 jobs for epirat-incorrect-http-impl-workaround in 1 minute and 22 seconds
Status Job ID Name Coverage
  Build
passed #10436
docker
build

00:00:27

 
  Test
passed #10438
docker
lint

00:00:52

passed #10437
docker
test

00:00:55

24.1%