Fixed wrong 'write_connection' variable name for models.

After upgrade to 1.7.2 I did not realise you had to set the `db_write_connection` variable. And so realised that the normal `$_connection' was getting overridden to `null` as I use a different DB for Auth stuff.

So I believe there is a miss type in the variable names, but correct me if I'm wrong.

PS: Also some how believe that if `ormauth.db_write_connection` is null, then it should use the ormauth.db_connecton' setting as default (instead of null), as we presume then its not different front he read connection. Right now it would pull from "default" which would be a different connection.