Making a request with the method request() will always override the content-type header, with application/x-www-form-urlencoded.

Example to reproduce the problem

 $client = static::createClient();
 $headers['content-type'] = 'some.other valid IANA';
 $client->request('POST', '/api/test', [], [], $headers, 'HELO');

the request sent to the controller will contain application/x--www-form-urlencoded

Also trying with:

$headers['HTTP_content-type'] = 'some.other valid IANA' same result