Fix EZP-21758: Custom variables are not correctly exposed in legacy templates when doing a sub-request
EZP-21758: Fixed array to string conversion
EZP-21758: Fixed another (stupid) array to string conversion

This was due to the fact that "_custom" key in $viewParameters was
removed after $viewParameters was assigned to the template.
Yes, (facepalm)...
Fix EZP-21656: PSpell plugin fails with eZFormToken