#7322 - i18n keys hashmap is not populated after save()

To reproduce:

```sell
php symfony test:unit issue7322
```

Mike Gale (@MikeFE) has a similar script to reproduce the problem: http://pastebin.com/ERbbcs5t.

It's actually an old bug, reproducible in ICA-AtoM 1.3.1 too.

- - -

This is the SQL generated when the title is updated:
```sql
UPDATE information_object_i18n
SET `TITLE`='Description 542b10d49ddb0', `ID`='389319'
WHERE information_object_i18n.ID IS NULL AND information_object_i18n.CULTURE IS NULL
```

It should be like the following:
```sql
UPDATE information_object_i18n
SET `TITLE`='Description 542b10d49ddb0', `ID`='389319'
WHERE information_object_i18n.ID='389319' AND information_object_i18n.CULTURE='en'
```

The problem seems to be in [BaseInformationObjectI18n.php#L522](https://github.com/artefactual/atom/blob/qa/2.2.x/lib/model/om/BaseInformationObjectI18n.php#L522):

```php
if ($column->isPrimaryKey())
{
  $selectCriteria->add($column->getFullyQualifiedName(), $this->keys[$column->getPhpName()]);
}
``` 

`$this->keys` happens to be empty, because it's not populated when the object is inserted/updated and new i18n rows are created.