Nested transaction: begin()+rollback() with nested transaction() does not rollback

[krasa]

begin(), commit() & rollback() calls are forbidden in transaction() callback, but the other way should be forbidden also, or supported - rollback() currently does not work silently.

relates to: #387

example:

$dibi = new Dibi\Connection([
  'driver' => 'postgre',
//  ...
]);

echo "<h2>Before</h2>\n";
$dibi->query('SELECT * FROM [products]')->dump();

$dibi->begin();

$dibi->transaction(function ($transaction) use ($dibi) {
  $dibi->query('INSERT INTO [products]', [
    'title' => 'Test product',
  ]);
});

$dibi->rollback();

echo "<h2>After rollback</h2>\n";
$dibi->query('SELECT * FROM [products]')->dump();
// -> rollback did not work

[dg]

You can send a PR with fix?