Nested joins do not check if already joined

[hkdobrev]

When you have a query builder like:

$query = Jam::all('a')
    ->join_nested('b')
        ->join('c')
    ->end();

and later on you have:

$query
    ->join_nested('b')
        ->join('c')
    ->end();

Jam would check that you have already joined "b", but it would not check that you have already joined "c". As a result it would trigger a MySQL error saying:

Not unique table/alias: 'c'