Returns the appropriate content type
This normally simply returns the contentType property but can be overridden by an extending class if the content-type is variable
Returns the serialized content of the response class
You can either fill the payload elsewhere in an extending class and leave this method alone, or you can override it if you have a different type of payload that needs special treatment
Overridden in child classes as:
- HTML_AJAX_Action::getPayload()
- Returns the serialized content of the response class