Send "Accept" request header to indicate RFC 9457 compliance

[kwin]

Also evaluate RFC 9457 responses for PUT requests
Add tests to check for proper "Accept" headers for GET/HEAD/PUT

This closes #1845

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[cstamas]

CF sends Content-Type: application/problem+json; charset=utf-8 on response, so equality will not work.

[cstamas]

Maybe just "contains" or "starts with"? (with proper nullcheck on contentType?). Just to keep it simple.

[kwin]

I strip attributes from that header now in e4521fa.