Add option for overriding root spec (#1317)

* Only for pg >= 9.6

* Disallow specifying schema on root-spec

* Increase memory test upper bound

test/fixtures/schema.sql

@@ -1680,3 +1680,36 @@ create function add_them(a integer, b integer)
 returns integer as $$
   select a + b;
 $$ language sql;
+
+create function root() returns jsonb as $_$
+declare
+openapi jsonb = $$
+  {
+    "swagger": "2.0",
+    "info":{
+      "title":"PostgREST API",
+      "description":"This is a dynamic API generated by PostgREST"
+    }
+  }
+$$;
+simple jsonb = $$
+  [
+    {
+      "table":"items"
+    },
+    {
+      "table":"subitems"
+    }
+  ]
+$$;
+begin
+case current_setting('request.header.accept', true)
+  when 'application/openapi+json' then
+    return openapi;
+  when 'application/json' then
+    return simple;
+  else
+    return openapi;
+  end case;
+end
+$_$ language plpgsql;