private String[] asString(Map<String, Collection<String>> headers) {
    return headers.entrySet().stream()
        .flatMap(
            entry ->
                entry.getValue().stream()
                    .map(value -> Arrays.asList(entry.getKey(), value))
                    .flatMap(List::stream))
        .toArray(String[]::new);
  }
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