pgsql: Fix overly enthusiastic Assert introduced in 8.1: it's expecting

Log Message:
-----------
Fix overly enthusiastic Assert introduced in 8.1: it's expecting a
CaseTestExpr, but forgot that the optimizer is sometimes able to replace
CaseTestExpr by Const.

Modified Files:
--------------
pgsql/src/backend/utils/adt:
ruleutils.c (r1.232 -> r1.233)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/ruleutils.c.diff?r1=1.232&r2=1.233)