Bug fixes for S4S Imaging Demo

[abjonnes]

We (Sync for Science) are developing a demo application of the imaging stack which runs in the browser. Part of the demo is fetching the FHIR ImageStudy data from the FHIR broker, which fails due to a Access-Control-Request-Headers: Authorization header in the pre-flight request from the browser. Adding Authorization to the allowed headers list in the CORS configuration allows the request to succeed.

See also a similar pull request in the DICOM-RS broker repository.

[abjonnes]

I added a few more commits to cover some other bugs we encountered. We want to use the forwarding capability in this project to forward the DICOM data from the DICOM-RS broker to the client but there were some issues:

• The headers were not being forwarded correctly (see the Java docs - the 0th header was the HTTP status line so the value of conn.getHeaderFieldKey(0) was null even though there were other headers)
• The DICOM-RS broker returns a 503 error when the data isn't ready, but this wasn't being handled well (an empty 200 response was being returned to the client)