[COMMITTERS] pgsql: Fix ruleutils.c for domain-over-array cases, too.

Fix ruleutils.c for domain-over-array cases, too.

Further investigation shows that ruleutils isn't quite up to speed either
for cases where we have a domain-over-array: it needs to be prepared to
look past a CoerceToDomain at the top level of field and element
assignments, else it decompiles them incorrectly.  Potentially this would
result in failure to dump/reload a rule, if it looked like the one in the
new test case.  (I also added a test for EXPLAIN; that output isn't broken,
but clearly we need more test coverage here.)

Like commit b1cb32fb6, this bug is reachable in cases we already support,
so back-patch all the way.

Branch
------
REL9_6_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/ff2d5372235dae339cbfa04aa55a8458cd6bfab8

Modified Files
--------------
src/backend/utils/adt/ruleutils.c    | 41 ++++++++++++++++++++++++++++++++----
src/test/regress/expected/domain.out | 41 ++++++++++++++++++++++++++++++------
src/test/regress/sql/domain.sql      | 11 +++++++++-
3 files changed, 81 insertions(+), 12 deletions(-)