public PayloadBuilder actionKey(final String actionKey) {
        customAlert.put("action-loc-key", actionKey);
        return this;
    }
Sets the value of action button (the right button to be
displayed).  The default value is "View".

The value can be either the simple String to be displayed or
a localizable key, and the iPhone will show the appropriate
localized message.

A {@code null} actionKey indicates no additional button
is displayed, just the Cancel button.

@param actionKey the title of the additional button
@return  this
src/main/java/com/notnoop/apns/PayloadBuilder.java