[fix] broken events before and after save on JModelAdmin. Related #4308

This PR broke event B/C https://github.com/joomla/joomla-cms/pull/4308

The default context passed to plugins on  `JModelAdmin` save was:

```php
$this->option . '.' . $this->name`
```

and it was replaced by:

```php
$context = $this->option . '_' . $this->name;
```

That breaks all the existing plugins triggered before and after save events.

See: 
https://github.com/joomla/joomla-cms/pull/4308/files#diff-39b4bc5cb02bbace2ee4febe8f54583eR1079