Missing location headers from webauthdemo

[FireJuun]

Bug

When using the web auth demo, particularly with an EHR client (Epic Clinician), we are currently unable to grab the relevant response header that is linked to patient IDs.

Steps to Reproduce

1. Checkout this repo, and setup local api keys per these instructions, placing them inside fhir/fhir_auth/webauthdemo/lib
2. Go into fhir/fhir_auth/webauthdemo and run the command ./run_locally.sh to start a locally-hosted Flutter web app
3. Inspect the web app, focusing on the Network tab
4. Click on Epic Clinician, then use the default clinician login per this site
5. The ResponseHeaders contained in Patient?_format=json&_pretty=false on the inspected web application should match the ResultHeaders that are shown in the terminal

Expected results:
Expect ResponseHeaders that include Location data (aka: patient ID), in web app inspection

Access-Control-Allow-Origin: *
Cache-Control: no-cache,no-store
Content-Length: 0
Content-Type: application/fhir+json; charset=utf-8
Date: ________________
Expires: -1
Location: Patient/________________
Pragma: no-cache
Server: Microsoft-IIS/10.0
...

Actual results:
ResultHeaders, shown in terminal

{cache-control: no-cache,no-store, content-length: 0, content-type:
application/fhir+json; charset=utf-8, expires: -1, pragma: no-cache}}]}