OCI: CLOBs never alter

@butonic 

This fixes that CLOB fields are automatically marked to alter. The flag is set by https://github.com/owncloud/3rdparty/blob/stable5/MDB2/Driver/Datatype/Common.php#L960 with regard to the Length. It's the text comparison method because it is called from https://github.com/owncloud/3rdparty/blob/stable5/MDB2/Driver/Datatype/Common.php#L987

One note: I did not test what happens when a column is changed from non-clob to clob and vice versa.