Fix collation safety rules for aggregations

For default input collation, inner_cxt.state is marked as
FDW_COLLATE_NONE.

There's the situation in which agg.collation == innert_cxt.collation
== DEFAULT_COLLATION_OID and thus it should be safe to push down the
aggregation when FDW_COLLATE_NONE and FDW_COLLATE_SAFE.

This fixes it by checking against FDW_COLLATE_UNSAFE.

Author: Rafa de la Torre

contrib/postgres_fdw/deparse.c

@@ -754,7 +754,7 @@ foreign_expr_walker(Node *node,
 				 */
 				if (agg->inputcollid == InvalidOid)
 					 /* OK, inputs are all noncollatable */ ;
-				else if (inner_cxt.state != FDW_COLLATE_SAFE ||
+				else if (inner_cxt.state == FDW_COLLATE_UNSAFE ||
 						 agg->inputcollid != inner_cxt.collation)
 					return false;