public PayloadBuilder customFields(final Map<String, ?> values) {
        root.putAll(values);
        return this;
    }
Set any application-specific custom fields.  These values
are presented to the application and the iPhone doesn't
display them automatically.

This method *adds* the custom fields in the map to the
payload, and subsequent calls add but doesn't reset the
custom fields.

@param values   the custom map
@return  this
src/main/java/com/notnoop/apns/PayloadBuilder.java