docs: private static boolean isApplicationHeader(String key) {
      // Don't allow reserved non HTTP/2 pseudo headers to be added
      // HTTP/2 headers can not be created as keys because Header.Key disallows the ':' character.
      return !CONTENT_TYPE_KEY.name().equalsIgnoreCase(key)
          && !USER_AGENT_KEY.name().equalsIgnoreCase(key)
          && !TE_HEADER.name().equalsIgnoreCase(key);
    }