Image::with_direct_io(): don't modify datatype of internal Buffer
Image::with_direct_io(): don't modify datatype of internal Buffer

This causes crashes on write-back for the rare cases where an _output_
image is accessed using .with_direct_io() (see e.g. #489).

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3