throw an error when _serialize not properly set.

prevents unhelpful error message when a CSV view is missing. Without the change you get the following:

Warning (2): Invalid argument supplied for foreach() [APP/Plugin/CsvView/View/CsvView.php, line 227]
Warning (2): Invalid argument supplied for foreach() [APP/Plugin/CsvView/View/CsvView.php, line 227]
Warning (2): Invalid argument supplied for foreach() [APP/Plugin/CsvView/View/CsvView.php, line 227]

when requesting a csv file for which there isn't a template.

Throwing this error allows CakePHP to report the real error.