pgsql: Don't let transform_null_equals=on affect CASE foo WHEN NULL ...

Don't let transform_null_equals=on affect CASE foo WHEN NULL ... constructs.
transform_null_equals is only supposed to affect "foo = NULL" expressions
given directly by the user, not the internal "foo = NULL" expression
generated from CASE-WHEN.

This fixes bug #6242, reported by Sergey. Backpatch to all supported
branches.

Branch
------
REL8_2_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/aadfd6e5d660f497be3f64fc120c4339da6c7192

Modified Files
--------------
src/backend/parser/parse_expr.c |    7 +++++--
1 files changed, 5 insertions(+), 2 deletions(-)