pgsql: Fix contrib/postgres_fdw's remote-estimate representation of arr

Fix contrib/postgres_fdw's remote-estimate representation of array Params.

We were emitting "(SELECT null::typename)", which is usually interpreted
as a scalar subselect, but not so much in the context "x = ANY(...)".
This led to remote-side parsing failures when remote_estimate is enabled.
A quick and ugly fix is to stick in an extra cast step,
"((SELECT null::typename)::typename)". The cast will be thrown away as
redundant by parse analysis, but not before it's done its job of making
sure the grammar sees the ANY argument as an a_expr rather than a
select_with_parens. Per an example from Hannu Krosing.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/5b68d81697bcb0d16136bd037e454ee53c521185

Modified Files
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contrib/postgres_fdw/deparse.c | 84 +++++++++++++++---------
contrib/postgres_fdw/expected/postgres_fdw.out | 19 ++++++
contrib/postgres_fdw/sql/postgres_fdw.sql | 3 +
3 files changed, 76 insertions(+), 30 deletions(-)