[cloud_functions] (WEB) Workaround to avoid preflight Request.

[kaumudpa]

While calling oncall http function from Web app there is always a CORS preflight request which ends with status 204 before the actual function starts.

Is there a workaround to avoid this? Since the preflight request invocation is also counted as a invocation towards the allotted quota.

To Reproduce:

1. Call any simple http function via cloud_functions plugin on flutter web.
2. Check the Firebase console, You will find 2 executions of that function - The first one ending with status204 and the second one ending with 200.

The first execution is the preflight request while the second one is the actual execution of our function.

Note : This behaviour is only shown while calling the function from flutter web. On IOS and Andriod the function works completely fine.

[TahaTesser]

cloud_functions: ^0.5.0

[noelmansour]

This seems to be an issue with the js sdk: See firebase/firebase-js-sdk#1800. You can find some workarounds there.