com_contact: Fix PHP strict warnings on sending email

When sending an email (that uses JFormRuleContact* Rules), PHP strict error appears:

`Strict standards: Declaration of JFormRuleContactEmail::test() should be compatible with JFormRule::test(SimpleXMLElement $element, $value, $group = NULL, JRegistry $input = NULL, JForm $form = NULL)`

Issue tracker: [#29107](http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=29107)

There was a PR by @elinw: #523, but it has been closed.