Narrow the substitution of = by IN in queries

## What I did
Created a virtual field with a subquery in the WHERE part. The subquery had a limit statement.

## What happened
SQLSTATE[42000]: Syntax error or access violation: 1235 This version of MySQL doesn't yet support 'LIMIT & IN/ALL/ANY/SOME subquery'

## What I expected to happen
There should have been no error, since the virtual field had no IN/ALL/ANY/SOME subquery. However, DboSource::fetchAssociated() (in lib/Cake/Model/Datasource/DboSource.php) had replaced the '= (' by 'IN ('. 

## Possible solution
Please restrict the replacement to the part for which it is meant, i.e. the part where the ids are inserted by Cake? E.g.:
```php
public function fetchAssociated(Model $model, $query, $ids) {
	if (count($ids) > 1) {
		$query = str_replace('= ({$__cakeID__$}', 'IN ({$__cakeID__$}', $query);
	}
	$query = str_replace('{$__cakeID__$}', implode(', ', $ids), $query);
	return $this->fetchAll($query, $model->cacheQueries);
}
```