Merge pull request #69 in ZP/z-push from bugfix/ZP-656-warnings-for-signed-emails-in-sent to develop

* commit '8139b02b':
  ZP-656 Only prepend transport headers if they are available. Do not include the newlines to separate the headers from the body, as the data already contains headers and the separating lines. If no transport headers are available in the case of a signed message do not use the signed attachment but let imtoinet() generate the email as it generates the headers by default. See ZP-725 for more information.