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

Log Message:
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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.

Tags:
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REL8_1_STABLE

Modified Files:
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    pgsql/src/backend/utils/adt:
        ruleutils.c (r1.207.2.5 -> r1.207.2.6)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/ruleutils.c.diff?r1=1.207.2.5&r2=1.207.2.6)