Correct line breaks in quoted-printable encoding.

Quoted-printable transfer encoding is intended to be "line break
neutral." It uses CRLF to encode line breaks, whether you prefer
carriage returns, line feeds, or both.

Ruby's builtin #pack and #unpack support quoted-printable encoding,
but expect to see linefeeds (\n) only. So it's our responsibility to
convert to LF before encoding and to CRLF afterward, and to convert
CRLF to LF after decoding.
Merge pull request #496 from jeremy/quoted-printable-line-break-neutrality

Correct line breaks in quoted-printable encoding