Bug 17842: UTF-8 encode ISO2709 MARC download from cart

Bug 17842: UTF-8 encode ISO2709 MARC download from cart

The cart was outputing ISO2709 MARC records with Latin-1
encoding. Records containing non-latin1 characters were
automatically re-encoded as UTF-8 by browsers, which led to
inconsistent character encodings for downloaded MARC files.

This patch explicitly encodes ISO2709 MARC characters from
the cart download as UTF-8 encoded bytes, which resolves the problem.

Test Plan:
0) Don't apply patch
1) Create bib record with only ASCII characters
2) Add a ü character to the title
3) Save bib record
4) Download bib record from cart (opac and staff client)
5) Using xxd or some other program, note that the ü is
represented by a FC byte (latin-1 encoded)
6) Apply the patch
7) Download bib record from cart (opac and staff client)
8) Using xxd or some other program, note that the ü is
represented by C3 BC bytes (utf-8 encoded)
9) Success

(Note that you could potentially use Notepad++ or some other
program to open the downloaded file and just note the encoding
that it finds. You could also try "chardetect" instead. Lots
of options for figuring out the encoding.)

Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>