countQuery() replaces connection information with active

The SelectQuery produced by SelectQuery::countQuery() replaces the connection information in the SelectQuery with the active connection (it uses db_select to form the new query, instead of the select() method of the original query's connection). This caused Drupal to attempt to run a SELECT COUNT(*) query on the wrong database when I was attempting to use Migrate module on an external database.

I marked this Needs Work because I would like someone wiser than I to verify that it should be
$this->connection->select($count) rather than
$count->connection->select($count)

I don't know that it matters, but it seems like it'd be good to check.