Bug 14592: Export data bad encoding

Bug 14592: Export data bad encoding

Tools > Export data produces a file badly encoded, when marc/xml file
format is chosen. It works with CSV.

To test:

1. Tools > Export data
2. Choose a limited interval of biblionumber
3. Export in marc/xml/csv.
4. Check files exported at 3: csv file is ok. marc/xml files are
   corrupted, ie badly encoded.
5. Apply the patch
6. Repeat steps 3&4, and confirm that all file formats are valid.

Signed-off-by: Tomas Cohen Arazi <tomascohen@unc.edu.ar>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit dfe07fa410eb48241c218abcc156c466576dcc9a)
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>