Change required MySQL database character set variable

Changing `character_set_server` requires root permissions and rebooting
the server.

Which is impossible on shared web hosting services.

So use `character_set_database`. This variable can be changed with
user permissions using `ALTER DATABASE`.

lib/util/installer.php

@@ -383,7 +383,7 @@ abstract class Installer
                 break;
             case 'mysql':
                 $res = $conn->query(
-                    "SHOW SESSION VARIABLES LIKE 'character_set_server'"
+                    "SHOW SESSION VARIABLES LIKE 'character_set_database'"
                 );
                 if (MDB2::isError($res)) {
                     throw new Exception($res->getMessage());