code: def _get_11paths_serialized_headers(x_headers):
      
      if x_headers:
          headers = to_key_val_list(x_headers, sort=True, insensitive=True)
          serialized_headers = ""
          for key, value in headers:
              if key.lower().startswith(X_11PATHS_HEADER_PREFIX.lower()) and \
                              key.lower() != X_11PATHS_DATE_HEADER_NAME.lower():
                  serialized_headers += key.lower() + X_11PATHS_HEADER_SEPARATOR + value.replace("\n", " ") + " "
          return serialized_headers.strip()
      else:
          return ""
docs: Prepares and returns a string ready to be signed from the 11-paths specific HTTP headers received.
  
      :param x_headers: a non necessarily ordered map (array without duplicates) of the HTTP headers to be ordered.
      :return: string The serialized headers, an empty string if no headers are passed, or None if there's a problem such
      as non 11paths specific headers