newheaders.put(SignatureHelper.USER_HEADER, "notification");
newheaders.put(SignatureHelper.TIMESTAMP_HEADER, "\"" + System.currentTimeMillis() + "\"");
//sign only those headers that are Strings
Map<String, List<String>> stringHeaders = new HashMap<String, List<String>>();
for (String key : headers.keySet()) {
if (headers.get(key) instanceof String) {
List<String> ss = new ArrayList<String>();
ss.add(headers.get(key, String.class));
stringHeaders.put(key, ss);
}