Improvements
* Allow single-part response message from origin server for the Retrieve Instance transaction. It is debatable whether this behaviour is compliant with the standard. Part 18 originally specified that an instance should be retrieved using content type `multipart/related; type="application/dicom"`, but Supplement 183 changed the wording of the standard and explicitly stated that `application/dicom` shall be used. This change in behavior was unintentional and [Correction Proposal 2040](https://www.dicomstandard.org/news-dir/current/docs/cpack108/cp2040.pdf) aims to correct this. However, since this behavior is allowed according to the current version of standard, we decided to support it.