Bug 27127: Make Koha::BackgroundJob->messages return a scalar

Bug 27127: Make Koha::BackgroundJob->messages return a scalar

Otherwise the code in the template is not handled correctly:
112                     [% FOR m IN job.messages %]

m will become each key/val of the message hash:
{"type":"success","biblionumber":"1","code":"biblio_modified"}
And not the whole hash

Test plan:
Modify one biblio using the batch record modification tool
Modify two biblios using the batch record modification tool
Go to the detail view of each job and confirm that with this patch the
display is correct

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>