origin 'null' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. code example

Example: been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource.

My issue was the remote server not responding to OPTIONS requests, 
so after fiddling about with requests and headers for what seemed 
like ages I resolved it by removing the headers 
Content-Type and Access-Control-Allow-Origin