After transforming a CASE expr with a default argument,

After transforming a CASE expr with a default argument,
delete the default argument from the node.  This prevents the executor
from spitting up on the untransformed argument expression.  Typical
failure was:
select (case f1 when 'val' then 'subst' else f1 end) from t1;
ERROR:  copyObject: don't know how to copy 704