CRM-12811 fix : passed 'GET' in $method argument for CRM_Utils_Request::retrieve which is being invoked from inside the function CRM_Core_BAO_CustomGroup::extractGetParams

CRM_Core_BAO_CustomGroup::extractGetParams function is used in Event online registration and online contribution forms specifically to set default values which are passed in URL as GET parameter.

The notices were thrown cause : 
 the checkbox element is initialized as array in $_POST, and since the CRM_Utils_Request::retrieve uses $method = 'REQUEST' by default, checkbox array in $_POST was getting processed.