[BUG] incorrect typing causes incorrect sorting

[RoyWilliams]

Describe the bug
Subtracting two double's should be a double not a char

To Reproduce

1. Go to Firefly portal
2. Choose TAP service = RSP
3. Choose ADQL
4. Enter this query and click Search
   SELECT mag_g,mag_r, mag_g-mag_r as color
   FROM dp01_dc2_catalogs.object
   WHERE CONTAINS(POINT('ICRS', ra, dec),CIRCLE('ICRS', 55, -35, 1))=1
   AND (mag_g <21 AND mag_r <21)

Expected behavior
If I click on the sort button in the display, it should sort as doubles, not as chars

Actual behavior
Seems strange that "color" is a char field.
If I click on the sort button in the display it sorts alphabetically instead of numerically

** Additional context **
However, if I add "ORDER BY color DESC" to my ADQL, then the sorting is correct.

[frossie]

Thanks for the report, we are investigating which is the offending layer of the sandwich

[fritzm]

Verified that Qserv is returning correctly typed result data to TAP for this query.

[frossie]

We have rolled out a fix to this. @RoyWilliams please give it a try.

[RoyWilliams]

Looks great! Thank you!

[frossie]

For the record, fix upstreamed to CADC, h/t @cbanek