Bug 17807: [Follow-up] Use marcxml for marc21 prefix

Not sure what happened here, but this is an easy fix.

Test plan:
Try oai.pl with marcxml, marc21 and oai_dc again.

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>

Koha/OAI/Server/Record.pm

@@ -43,7 +43,7 @@ sub new {
 
     my $format = $args{metadataPrefix};
     my $record_dom;
-    if ( $format ne 'marcxml' ) {
+    if ( $format ne 'marcxml' && $format ne 'marc21' ) {
         my $args = {
             OPACBaseURL => "'" . C4::Context->preference('OPACBaseURL') . "'"
         };