private Map<String, Collection<String>> castMapCollectType(Map<String, List<String>> map) {
    final Map<String, Collection<String>> result = new HashMap<>();
    map.forEach((key, value) -> result.put(key, new HashSet<>(value)));
    return result;
  }
There is a bunch o headers that the http2 client do not allow to be set.

@see jdk.internal.net.http.common.Utils.DISALLOWED_HEADERS_SET
java11/src/main/java/feign/http2client/Http2Client.java