This method allows you to append extra headers to the ones generated by
autoheaders, and should be used in place of the DkimSignature equivalents
when the Signer has autoHeaders switched on.
If autoHeaders is on (true), then we will ask the canoncialiser to provide us
with the list of headers present in the message, and recommended by the RFC.
Set the canonicalisation method which should be used for processing the
body. Throw a DkimError if we are a DomainKey signature, because you
can't specify defferent encodings for header and body when using DomainKey
Set the canonicalisation method which should be used for processing the
headers. Throw a DkimError if we are a DomainKey signature, because you
can't specify defferent encodings for header and body when using DomainKey