veredise audit fix: bh= may occur in other DKIM tags

[jp4g]

Addresses Veridise Audit Issue: "bh= may occur in other DKIM tags"

Solution:

1. adds a check for prefix of "; bh=" instead of "bh=" when accessing body hash from dkim-header field
2. adds a check for suffix ";" (no suffix check previously) when accessing body hash from dkim-header field

Includes a test with crafted dkim-signature field that should otherwise have passed old get_body_hash check but now fails

Stack order: 1