We can exploit eg: knowing that the list of headers is a unique
list (no repeated headers) and save some overhead when using the
same signer multiple times.
This help transitions to a cheaper email parser, that doesn't need to
parse the entire MIME structure
Note that as part of this, I found that several tests were running
against email messages using unix line endings instead of canonical
CRLF.
This was silently being masked and an empty or otherwise incorrect byte
slice was being hashed; I've updated the affected tests and their test
expectations accordingly, as well as made this error bubble up at the
time of parsing the email, rather than silently squashing it in the
depths of the hasher.
Wez Furlong