Cache directory for DiscISAM cache storage cannot be set

[AndreKR]

In https://github.com/PHPOffice/PHPExcel/blob/develop/Classes/PHPExcel/CachedObjectStorageFactory.php#L103-104 the dirparameter of the DiscISAM method is set to NULL.

In https://github.com/PHPOffice/PHPExcel/blob/develop/Classes/PHPExcel/CachedObjectStorageFactory.php#L199-204 only parameters for which isset() is true are set. isset(NULL)is false, so the dir parameter can never be set.

[MarkBaker]

modified isset() test to use array_key_exists() instead

[dalecosp]

Thanks Andre and Mark! Was definitely causing an issue in some backend production systems here at OMBE.