Correctly handle array pseudotypes in to_json and to_jsonb

Columns with array pseudotypes have not been identified as arrays, so
they have been rendered as strings in the json and jsonb conversion
routines. This change allows them to be rendered as json arrays, making
it possible to deal correctly with the anyarray columns in pg_stats.

src/test/regress/sql/json.sql

@@ -102,6 +102,12 @@ FROM rows q;
 
 SELECT row_to_json(row((select array_agg(x) as d from generate_series(5,10) x)),false);
 
+-- anyarray column
+
+select to_json(histogram_bounds) histogram_bounds
+from pg_stats
+where attname = 'tmplname' and tablename = 'pg_pltemplate';
+
 -- to_json, timestamps
 
 select to_json(timestamp '2014-05-28 12:22:35.614298');