ST_Union(rast, uniontype) treat bands with different classes as the same as first band

If the $RASTER has bands like ARRAY['8BUI', '16BUI'], the result of

```

            ST_Union(
              $RASTER,
              ARRAY[
                ROW(1, 'SUM'),
                ROW(2, 'MAX')
              ]::unionarg[]
            )

```

will be like ARRAY['8BUI', ' 8BUI'].

ST_Union always uses the first band class to process the 2nd band and ignore the original class.