cakephp/cakephp — pull request #933 by mrmorris (2012-11-02 13:38:25+00:00)
Title: Update checkRequired to use array_key_exists rather than isset

Not sure if this was intentional, but I found that I could not set a field's validation as:
required=>true
allowEmpty=>true
And have it pass validation when the value was present in the data but null.

isset() will return false when the value is null, so I switched this for array_key_exists in CakeValidationRule::checkRequired