Alumna::SMTP::QuotedPrintable

Quoted-printable body encoder (RFC 2045, section 6.7). Writes straight to the IO. It does not build a String.

Rules:

• CRLF, LF, and a lone CR in the input become one CRLF (a hard line break).
• Printable ASCII is literal, except "=".
• Space and tab are literal, except before a line break or at the end (then =20 / =09).
• Any other byte is =XX (upper-case hex). UTF-8 text becomes =XX triplets.
• A "." at the start of a line is =2E. So no output line starts with ".", and the SMTP DATA phase needs no dot-stuffing.
• A line has at most 75 characters, then a soft break ("=" CRLF). A soft break never splits an =XX triplet.

The output always ends at the start of a line (CRLF last), or is empty.