postgres_fdw: When sending ORDER BY, always include NULLS FIRST/LAST.

Previously, we included NULLS FIRST when appropriate but relied on the
default behavior to be NULLS LAST.  This is, however, not true for a
sort in descending order and seems like a fragile assumption anyway.

Report by Rajkumar Raghuwanshi.  Patch by Ashutosh Bapat.  Review
comments from Michael Paquier and Tom Lane.

contrib/postgres_fdw/deparse.c

@@ -2308,6 +2308,8 @@ appendOrderByClause(List *pathkeys, deparse_expr_cxt *context)
 
 		if (pathkey->pk_nulls_first)
 			appendStringInfoString(buf, " NULLS FIRST");
+		else
+			appendStringInfoString(buf, " NULLS LAST");
 
 		delim = ", ";
 	}

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -677,6 +677,18 @@ commit;
 select c2, count(*) from ft2 where c2 < 500 group by 1 order by 1;
 select c2, count(*) from "S 1"."T 1" where c2 < 500 group by 1 order by 1;
 
+-- Above DMLs add data with c6 as NULL in ft1, so test ORDER BY NULLS LAST and NULLs
+-- FIRST behavior here.
+-- ORDER BY DESC NULLS LAST options
+EXPLAIN (VERBOSE, COSTS false) SELECT * FROM ft1 ORDER BY c6 DESC NULLS LAST, c1 OFFSET 795 LIMIT 10;
+SELECT * FROM ft1 ORDER BY c6 DESC NULLS LAST, c1 OFFSET 795  LIMIT 10;
+-- ORDER BY DESC NULLS FIRST options
+EXPLAIN (VERBOSE, COSTS false) SELECT * FROM ft1 ORDER BY c6 DESC NULLS FIRST, c1 OFFSET 15 LIMIT 10;
+SELECT * FROM ft1 ORDER BY c6 DESC NULLS FIRST, c1 OFFSET 15 LIMIT 10;
+-- ORDER BY ASC NULLS FIRST options
+EXPLAIN (VERBOSE, COSTS false) SELECT * FROM ft1 ORDER BY c6 ASC NULLS FIRST, c1 OFFSET 15 LIMIT 10;
+SELECT * FROM ft1 ORDER BY c6 ASC NULLS FIRST, c1 OFFSET 15 LIMIT 10;
+
 -- ===================================================================
 -- test check constraints
 -- ===================================================================