Bug 8626: Fix encoding in cart emails for use of quoted-printable

Bug 8626: Fix encoding in cart emails for use of quoted-printable

When using quoted-printable data needs to be encoded accordingly.

Content-Transfer-Encoding: quoted-printable

Without the patch = in URLs or data will be interpreted as the beginning
of a special character resulting in broken URLs.

To test:
- Check diacritics in records display correctly
- Check various URLs with = to make sure they all work correctly

Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>
This is only a partial fix, as the subject line is not encoded,
but it is a huge improvement over what it was before.

Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>