only select database if it's not the default

When using eredis with sentinel (which implements the normal redis protocol) you get the following crash:

```
{unexpected_data,{ok,<<"-ERR unknown command 'SELECT'\r\n">>}}
```

This is because sentinel doesn't implement `SELECT`. This PR makes it so that `select_database` is only actually called if the database isn't the default, which fixes the problem in a backwards compatible way.