Calculate HTTP POST data in separate method for debugging

To check if the renderer works correctly it is desired to be able to
separate the test of the renderer and the function of the extension
PHP code. Therefore the method getPostData can be used to get
the values can be sent to the renderer directly via
curl –d 'POSTDATA' URL

Change-Id: I530f5fcba46364458643914bca81cf0bc1b60a58