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.