fix: Avoid casting to table type when select= and media type handler are used

Previously using a generic mimetype handler failed when any kind of select= was given, because
we tried to cast the select-result to the original table type. With this change, this cast is
only applied when select=* is given implicitly or explicitly. This is the only case where this
makes sense, because this guarantees that correct columns are selected in the correct order for
this cast to succeed.

Resolves #3160

test/spec/fixtures/schema.sql

@@ -3550,8 +3550,8 @@ returns "application/vnd.geo2+json" as $$
   select (jsonb_build_object('type', 'FeatureCollection', 'hello', 'world'))::"application/vnd.geo2+json";
 $$ language sql;
 
-drop aggregate if exists test.geo2json_agg(anyelement);
-create aggregate test.geo2json_agg(anyelement) (
+drop aggregate if exists test.geo2json_agg_any(anyelement);
+create aggregate test.geo2json_agg_any(anyelement) (
   initcond = '[]'
 , stype = "application/vnd.geo2+json"
 , sfunc = geo2json_trans
@@ -3755,3 +3755,16 @@ create aggregate test.some_agg (some_numbers) (
 
 create view bad_subquery as
 select * from projects where id = (select id from projects);
+
+-- custom generic mimetype
+create domain "pg/outfunc" as text;
+create function test.outfunc_trans (state text, next anyelement)
+returns "pg/outfunc" as $$
+  select (state || next::text || E'\n')::"pg/outfunc";
+$$ language sql;
+
+create aggregate test.outfunc_agg (anyelement) (
+  initcond = ''
+, stype = "pg/outfunc"
+, sfunc = outfunc_trans
+);