Subject REST call incorrectly serializes xsi type of subject assessors when multiple types exist

To recreate this:

1. Create a new project called TEST_PROJECT. You can also use an arbitrary project ID, you'll just need to substitute it on the REST call.
2. Upload both attached "sessions" via the compressed uploader (they're each just 1 frame from some sample data that I've modified).
3. Grab the JSON response for the subject at: /data/projects/TEST_PROJECT/subjects/SAMPLE_SUBJECT_NAME?format=json

If you search for "SessionData", you will find that the entries for the sessions will either be both "xsi:type": "xnat:mrSessionData" or both "xsi:type": "xnat:petSessionData". Which one "wins" out seems to be which session type was uploaded first. If you add a non-session subject assessor at the end, it also gets called an MR/PET session in this JSON response as well.