@Override public final Map<String, List<String>> getRequestProperties() {
    if (connected) {
      throw new IllegalStateException(
          "Cannot access request header fields after connection is set");
    }

    return OkHeaders.toMultimap(requestHeaders.build(), null);
  }
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