Fix for an error with CakeResque

- An array is passed to formatArgument() within Redisent.php, where an error is generated each time strlen() is run on the array

- Also mentioned in kamisama/Cake-Resque#30 by krazp

- This change simply checks whether the parameter is an array, if yes, it adds up the lengths of each value in the array

- If it doesn't get an array, it simply checks the length expecting a string