Bug 21056: Changing the logged in library can fail sporadically

We are getting reports that sometimes changing the logged in library via selectbranchprinter.pl sometimes appears to fail. Flushing the session after updating the branch appears to fix this.

Test Plan:
1) Change your logged in library repeatedly. Hopefully ( sort of ) it will fail to change.
   You may want to try various browsers.
2) Apply this patch
3) Note the change no longer fails

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

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

circ/selectbranchprinter.pl

@@ -62,6 +62,7 @@ if ( $branch and my $library = Koha::Libraries->find($branch) ) {
         $template->param(LoginBranchcode => $branch);       # update template for new branch
         $session->param('branchname', $branchname);         # update sesssion in DB
         $session->param('branch', $branch);                 # update sesssion in DB
+        $session->flush();
         push @updated, {
             updated_branch => 1,
                 old_branch => $userenv_branch,