'com_contact' bad words fix

This patch fixes 'com_contact' in that it does not obey the 'Banned Email, Banned Subject and Banned Text'.

A patch was submitted a submitted last year but for some reason it was
either reverted or not implemented.

Joomla Code tracker URL
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=28368