Properly set Pixel Presentation

Pixel Presentation != Pixel Representation

For reference:

* http://dicom.nema.org/medical/dicom/current/output/chtml/part03/sect_C.8.16.2.html#table_C.8-131

src/highdicom/legacy/sop.py

@@ -189,7 +189,10 @@ def _convert_legacy_to_enhanced(
         unique_image_types.add(tuple(frame_type))
         frame_type_item = Dataset()
         frame_type_item.FrameType = frame_type
-        frame_type_item.PixelRepresentation = pixel_representation
+        if pixel_representation == 0:
+          frame_type_item.PixelPresentation = 'MONOCHROME'
+        else:
+          frame_type_item.PixelPresentation = 'COLOR'
         frame_type_item.VolumetricProperties = volumetric_properties
         if frame_type[0] == 'ORIGINAL':
             frame_type_item.VolumeBasedCalculationTechnique = 'NONE'