@Override public final Map<String, List<String>> getHeaderFields() {
    try {
      return OkHeaders.toMultimap(getHeaders(),
          StatusLine.get(getResponse().getResponse()).toString());
    } catch (IOException e) {
      return Collections.emptyMap();
    }
  }
Returns the value of the field corresponding to the {@code fieldName}, or
null if there is no such field. If the field has multiple values, the
last value is returned.
gateway/platforms/servlet/src/main/java/io/apiman/gateway/platforms/servlet/connectors/ok/HttpURLConnectionImpl.java