veridise audit fix: first bh tag

[jp4g]

Addresses comment to bh= may occur in other DKIM tags

This will remove the issue. However, in the case that the very *first* DKIM-signature is `bh=`, it will fail. This may deny service to valid e-mails.

The case of `bh=` being the first DKIM tag should be handled.

Adds a check for : bh= or :bh= as valid prefixes to found body hashes in dkim-signature. Includes test_body_hash_at_start as test demonstrating that this case does not deny service to email verification.

Audit stack: 2