Improve CORS headers

[Syndesi]

CORS headers evolved to a point where * wildcards are no longer compatible with all domains / variants.
Instead, CORS headers usually must explicitly list all headers etc. which are used in later requests.

Acceptance criteria:

• Check which headers are currently being used.
• Write an integration test, which collects all header types being received/sent, and compares them against a list of white listed headers.
• Make sure that all actively used headers are defined in CORS header responses.