useCodeGrantFlow calls xhr.open twice (throws 'Cannot open, already sending')

[yonahforst]

when i comment out this line, it works: https://github.com/aws/amazon-cognito-auth-js/blob/808e304aac9f61b4a1e8d9af0f8f5196d32f3ce8/src/CognitoAuth.js#L541

(using it in a react native app)