fix: only regex a string

[twhitbeck]

For Shopify webhooks, authorization header is not present. We were trying to scrub nil by running it through a regex, which blows up.

[vasspilka]

Looks good to me. But I'm wondering wether we should just return nil when it's not binary instead of the scrub_value. Overall it seems to be specific to the "authentication" metadata key if we don't have it do we even want to set it to the scrub value?