FIX: Fix VirtualPage::init() content-modification check.
FIX: Fix VirtualPage::init() content-modification check.

This check had never worked but PHP 5 silently ignored it and PHP 7
raised it as an error.
dhensby/silverstripe-cms,silverstripe/silverstripe-cms,silverstripe/silverstripe-cms,dhensby/silverstripe-cms,dhensby/silverstripe-cms