code: public Map<String, List<String>> getAllAssetsMetadata() throws IOException, RequestFailureException {
          HttpURLConnection connection = createHeadConnection("/assets");
          testResponseCode(connection);
          return connection.getHeaderFields();
      }
docs: This method will issue a HEAD to all of the assets in massive and returns the {@link HttpURLConnection#getHeaderFields()}.
  
  @return The {@link HttpURLConnection#getHeaderFields()} for the HEAD request
  @throws IOException
  @throws RequestFailureException if the response code is not OK