Importing content and preserving old IDs fails.

Running Bolt 2.1.5 and PHP 5.5.8 and Apache 2.4.7 (on Windows - local development env.)

Trying to import content and preserving content ("post_id: id" in importwxr.bolt.yml) causes a fatal error:

> Bolt \ Exception \ StorageException
> Attempted to update a non-existent record

The relevant code:

```
// Test that the record exists in the database
$oldContent = $this->findContent($tablename, $content['id']);
if (empty($oldContent)) {
throw new StorageException('Attempted to update a non-existent record');
}
```

This probably worked with an older version of Bolt.