tap: allow an array of values for an audit exception

[bevanjkay]

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Discovered in Homebrew/homebrew-cask#191078
The audit fails due to a specific HTTP error, on two URLs within the one cask.
Presently, it isn't possible to pass two URLs to a single token via an array in the audit_exception list because the value isn't read.

This PR correctly parses the array.

[MikeMcQuaid]

Thanks @bevanjkay!