Ensure correct transfer encoding (force quoted-printable or base64) where required by RFCs

[skalee]

Extracted from #23.

RFC1847 section 2.1 says:

In addition, if the multipart/signed object is EVER to be transferred over the standard Internet SMTP infrastructure, the resulting MIME body is constrained to 7 bits -- that is, the use of material requiring either 8bit or binary content-transfer-encoding is NOT allowed. Such 8bit or binary material MUST be encoded using either the quoted-printable or base64 encodings.

This requirement exists because it is not generally possible, given the current characteristics of Internet SMTP, for a message originator to guarantee that a message will travel only along paths capable of carrying 8bit or binary material.

SMTP clients normally have the option of either converting the message to eliminate the use of 8bit or binary encoding or returning a nondelivery notification to the originator. However, conversion is not viable in the case of signed objects since conversion would necessarily invalidate the signature. This leaves a nondelivery notification as the only available option, which is not acceptable.

Furthermore, RFC3156 section 3 says:

Note that this also includes the case where a signed object is also encrypted (see section 6). This restriction will increase the likelihood that the signature will be valid upon receipt.