Multiple conditions by subselects are skipped

If I call multiple times 

```
$context->table('product')
      ->where('product.id', $selection1)
      ->where('product.id', $selection2);
```

only first condition is used and second is skipped.

Its caused by [this line of SqlBuilder](https://github.com/nette/database/blob/master/src/Database/Table/SqlBuilder.php#L216), as json_encode convers selection to '{}' string.
