fix: headerBasedAuth not updating the headers with new values

[chrisharrison]

Issue #, if available:

#666

Description of changes:

Allows the headerBasedAuth function to actually update the headers with the new value.

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[chrisharrison]

This fixes #666

[chrisharrison]

This should be applied as a hotfix to aws-appsync-auth-link version 2.0.5

[ronnyroeller]

The Amplify docs still advises to use this repo when using Apollo Client: https://docs.amplify.aws/lib/graphqlapi/upgrade-guide/q/platform/js/#option-2-upgrade-to-apollo-client-with-appsync-links

Wouldn't it make sense to merged this PR then? Or is there by now an alternative advised approach of how to use Apollo Client with AppSync?