return multipart for wado-rs instance

[pieper]

See discussion here:

ImagingDataCommons/dicomweb-client#41

Looks like we should be following this older version of the standard where it's stated clearly: http://dicom.nema.org/medical/dicom/2018e/output/chtml/part18/sect_6.5.3.2.html

[hackermd]

Based on my understanding, the retrieve study, series, and instance transactions of the DICOM web studies service shall serve "multiplart/related; type="application/dicom". At least this is how it is intended. Unfortunately, the refactoring of the standard via Supplement 183 created a lot of confusion regarding media types and it introduced application/dicom as a media type the retrieve instance transaction. This was unintentional and can be considered a "bug". WG 27 is working on addressing the issue and clarifying the text.