Application Link Status page fails to load due to Case-sensitive checks for Headers

Issue Summary

Trying to configure a new application link or loading the Application link page to list the status of existing Applinks will show "NETWORK ERROR" in red.
The REST client while reading headers (eg. Content-Type) does a case-sensitive lookup for them (headers name should be case-insensitive). So when headers are in lower case it can't parse them properly. The problem is triggered by the proxy server, which is converting all HTTP header names to lower-case.

Steps to Reproduce

1. Setup two Atlassian applications behind a proxy
2. Setup HAProxy with HTX enabled on one of the applications (2.x has it active by default)
3. Configure the Application link between the two applications

Expected Results

The application link can be configured and established

Actual Results

The application link is not configured correctly with "NETWORK ERROR" due to wrong headers.

The below exception is thrown in the atlassian-fisheye-<timestamp>.log file:

 WARN  [qtpxxxxxxx-xxx ] com.atlassian.applinks.core.DefaultApplinkStatusService DefaultApplinkStatusService-logApplinkError - Network error while attempting to retrieve status of Application Link '<applink-id>'

No further logging is available in DEBUG or TRACE. Wire logging or HAR file is required to capture the headers.

Workaround

Disable the headers casing in your load balancer

• For HAProxy 2.1 and newer:

  global
          h1-case-adjust content-type             Content-Type
          h1-case-adjust x-arequestid             X-AREQUESTID
          h1-case-adjust x-xss-protection         X-XSS-Protection
          h1-case-adjust x-content-type-options   X-Content-Type-Options
          h1-case-adjust x-asen                   X-ASEN
          h1-case-adjust x-ausername              X-AUSERNAME
  
  defaults
          option h1-case-adjust-bogus-client
  

• for Envoy - header_casing