Bug 6132: Update sysprefs cache in set_preference

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>

C4/Context.pm

@@ -514,7 +514,7 @@ the sysprefs cache.
 
 sub set_preference {
     my $self = shift;
-    my $var = shift;
+    my $var = lc(shift);
     my $value = shift;
 
     my $dbh = C4::Context->dbh or return 0;
@@ -530,7 +530,9 @@ sub set_preference {
         ON DUPLICATE KEY UPDATE value = VALUES(value)
     " );
 
-    $sth->execute( $var, $value );
+    if($sth->execute( $var, $value )) {
+        $sysprefs{$var} = $value;
+    }
     $sth->finish;
 }