isis2std bittype=s16bit exports a tiff with Type=UInt16

[ryodohemmi]

ISIS version(s) affected: 5.0.1

Description
isis2std bittype=s16bit outputs UInt16 (16-bit unsigned integer) tif, not Int16 (signed) tif.

How to reproduce
Input: Int16 cube

$ gdalinfo h5851_0002_sr2.cal2.cub
Driver: ISIS3/USGS Astrogeology ISIS cube (Version 3)
Files: h5851_0002_sr2.cal2.cub
Size is 1008, 1018
Corner Coordinates:
Upper Left  (    0.0,    0.0)
Lower Left  (    0.0, 1018.0)
Upper Right ( 1008.0,    0.0)
Lower Right ( 1008.0, 1018.0)
Center      (  504.0,  509.0)
Band 1 Block=1008x1018 Type=Int16, ColorInterp=Undefined
  NoData Value=-32768
  Mask Flags: 
  Metadata:
    BANDWIDTH=250.000000
    BANDWIDTH_UNIT=nm
    WAVELENGTH=600.000000
    WAVELENGTH_UNIT=nm

Running isis2std bittype=s16bit and its result

$ isis2std from=h5851_0002_sr2.cal2.cub to=h5851_0002_sr2.cal2.tif format=tiff bittype=s16bit
Group = Results
  OutputFileName = h5851_0002_sr2.cal2.tif
  InputMinimum   = 726.0
  InputMaximum   = 1602.0
End_Group

$ gdalinfo h5851_0002_sr2.cal2.tif
Driver: GTiff/GeoTIFF
Files: h5851_0002_sr2.cal2.tif
Size is 1008, 1018
Coordinate System is `'
Image Structure Metadata:
  INTERLEAVE=BAND
Corner Coordinates:
Upper Left  (    0.0,    0.0)
Lower Left  (    0.0, 1018.0)
Upper Right ( 1008.0,    0.0)
Lower Right ( 1008.0, 1018.0)
Center      (  504.0,  509.0)
Band 1 Block=1008x1 Type=UInt16, ColorInterp=Gray

Additional context
Running isis2std bittype=u16bit results in a UInt16 tif, which looks fine.

$ isis2std from=h5851_0002_sr2.cal2.cub to=h5851_0002_sr2.cal2.tif format=tiff bittype=u16bit
Group = Results
  OutputFileName = h5851_0002_sr2.cal2.tif
  InputMinimum   = 726.0
  InputMaximum   = 1602.0
End_Group

$ gdalinfo h5851_0002_sr2.cal2.tif
Driver: GTiff/GeoTIFF
Files: h5851_0002_sr2.cal2.tif
Size is 1008, 1018
Coordinate System is `'
Image Structure Metadata:
  INTERLEAVE=BAND
Corner Coordinates:
Upper Left  (    0.0,    0.0)
Lower Left  (    0.0, 1018.0)
Upper Right ( 1008.0,    0.0)
Lower Right ( 1008.0, 1018.0)
Center      (  504.0,  509.0)
Band 1 Block=1008x1 Type=UInt16, ColorInterp=Gray

The similar issue was reported in the past for both tiff and jpeg2000 formats: #1735
The signed 16 bit format is commented out in the source code:
https://github.com/USGS-Astrogeology/ISIS3/blob/7.0/isis/src/base/apps/isis2std/tsts/tiff/Makefile