Improve CORS handling for requests responded with errors

[gcotelli]

• Use the available CORS configuration to set the expected headers in case the HTTPClientError handler kicked-in.
• Fix the global error handler configured in StargateApplication to pay attention to CORS.
• Improve tests related to CORS.

Fixes #180

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a1ca293) 99.53% compared to head (c045270) 99.54%.

Additional details and impacted files

@@                  Coverage Diff                  @@
##           release-candidate     #181      +/-   ##
=====================================================
+ Coverage              99.53%   99.54%   +0.01%     
=====================================================
  Files                    164      166       +2     
  Lines                  13002    13306     +304     
=====================================================
+ Hits                   12941    13245     +304     
  Misses                    61       61              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.