API: Send Content-Type as api response header too

[yhabteab]

resolves #6747

[Al2Klimov]

Write a test protocol.

[yhabteab]

Testprotocol:
# curl -i -X OPTIONS -H 'Origin: https://localhost' -H 'Accept: application/json' -H 'Access-Control-Request-Headers: authorization,content-type,x-http-method-override' -H 'Access-Control-Request-Method: POST' -k -u 'root:icinga' 'https://localhost:5665/v1/objects/services'

Before:

HTTP/1.1 200 OK
Server: Icinga/v2.12.0-rc1-52-g04704a49a
Access-Control-Allow-Credentials: true
Access-Control-Allow-Methods: GET, POST, PUT, DELETE
Access-Control-Allow-Headers: Authorization, X-HTTP-Method-Override
Content-Length: 12
Connection: close

Preflight OK

After:

HTTP/1.1 200 OK
Server: Icinga/v2.12.0-rc1-52-g04704a49a
Access-Control-Allow-Credentials: true
Access-Control-Allow-Methods: GET, POST, PUT, DELETE
Access-Control-Allow-Headers: Authorization, Content-Type, X-HTTP-Method-Override
Content-Length: 12
Connection: close

Preflight OK