Buffer deprecation warning

[tim-lai]

Content & configuration

discovered during existing Jest unit test (same with Mocha): test/mocha/core/oauth2-authorize.js

Is your feature request related to a problem?

(node:19734) [DEP0005] DeprecationWarning: Buffer() is deprecated due to security and usability issues. Please use the Buffer.alloc(), Buffer.allocUnsafe(), or Buffer.from() methods instead.

Describe the solution you'd like

In /src/core/utils.js -> btoa method, ~line 600. Note that this line was a fix too, so investigate and confirm update. Otherwise, this is the only instance of Buffer in the project)

Describe alternatives you've considered

Additional context