Use GMT localtime only if localzone is not set

If we try to get the current date using JDate, for example:

```
$date = new JDate();
$date->format('H:i:s')
```
This example return the current time but using the GMT timezone and not using the saved into the Joomla! configuration (in my case 'America/Argentina/Buenos_Aires' GMT-3).

With the current patch, this bug is fixed.