Fix usage of Sequel::SQL::Blob objects as prepared statement arguments in jdbc/db2 adapter when use_clob_as_blob is false
Make use_clob_as_blob false by default on DB2

This was only true for historical reasons, because blob
columns originally didn't work well when DB2 support was first
added.  With the previous commit the specs run fine with
use_clob_as_clob = false, so for consistency with other
adapters I'm switching the default.
Don't return clob column values as SQL::Blob instances in the db2 and ibmdb adapters unless use_clob_as_blob is true