pgsql: Fix pg_dump assertion failure when dumping pg_catalog.

Fix pg_dump assertion failure when dumping pg_catalog.

Commit 396d348b04 did not account for the default collation.

Also, use pg_log_warning() instead of Assert().

Discussion: https://postgr.es/m/ce071503fee88334aa70f360e6e4ea14d48305ee.camel%40j-davis.com
Reviewed-by: Michael Paquier
Backpatch-through: 15

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/37188cea0c16f3f27ec65b526ffe12f6469142fc

Modified Files
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src/bin/pg_dump/pg_dump.c        | 69 ++++++++++++++++++++++++++++++----------
src/bin/pg_dump/t/002_pg_dump.pl |  8 +++++
2 files changed, 61 insertions(+), 16 deletions(-)