title: CORS issue in ionic4
body: <p>I am using Angularfire 2 and calling cloud function from my ionic 4 app.How to solve the  following CORS issue in ionic 4 ??</p>
  
  <p>Access to fetch at '<a href="https://null-myionicshop-693bc.cloudfunctions.net/subscribeToTopic" rel="nofollow noreferrer">https://null-myionicshop-693bc.cloudfunctions.net/subscribeToTopic</a>' from origin '<a href="http://localhost:8100" rel="nofollow noreferrer">http://localhost:8100</a>' 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. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.</p>