Fix issue with refresh tokens in the Dropbox sample

[erickoledadevrel]

See the Dropbox OAuth documentation. Fixes #434.

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[conventional-commit-lint-gcf[bot]]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot https://conventionalcommits.org/

[surusaki8]

Thank you very much for your help. I can now make an API request after a few hours and it returns the value I expected. I think I have probably solved the problem.

[erickoledadevrel]

@sqrrrl Friendly ping.