TAXII 2.1 collection request Content-Type mismatch with client

[chorsley]

Current behaviour: fetching collections returns a Content-Type header of application/stix+json.
Expected behaviour: fetching collections should return a Content-Type header of application/taxii+json.

Assume a simple TAXII 2.1 client connecting to attack-taxii.mitre.org using https://github.com/oasis-open/cti-taxii-client and this code:

from taxii2client.v21 import Server
server = Server('https://attack-taxii.mitre.org/api/v21/')

api_root = server.api_roots[0]
print(api_root.collections)

This code returns the following error:

taxii2client.exceptions.TAXIIServiceException: Unexpected Response. Got Content-Type: 'application/stix+json; charset=utf-8; version=2.1' for Accept: 'application/taxii+json;version=2.1'

This seems to have passed testing because looking at the test code at https://github.com/mitre-attack/attack-workbench-taxii-server/blob/d5334fa6d3de3befeb6ac6329a8d57bf19c4653c/bruno/Get%20A%20Collection.bru, it sends application/taxii+json in the TAXII HTTP request as an accepted response content-type:

headers {
  Accept: application/taxii+json;version=2.1
}

But later checks for application/stix+json in the response instead:

  test("Verify response content type", function() {
    const contentType = res.getHeader('content-type');
    expect(contentType).to.include('application/stix+json');
    expect(contentType).to.include('version=2.1');
  });

The TAXII 2.1 spec also shows it returning application/taxii+json for such requests at https://docs.oasis-open.org/cti/taxii-2.1-interop/v1.0/csd01/taxii-2.1-interop-v1.0-csd01.html#_j0vnatwbha29.