nested form for relation

[Maxim-Chugaev]

https://github.com/adamwathan/form/blob/master/src/AdamWathan/Form/Binding/BoundData.php#L67

 return $this->dataGet($target->{$key}, $keyParts, $default);

This is not worked for collection in model relations, such as many to many or many to one. i think need this change to $target[$key]

[cirdog]

#113 Just try my solution.

The Author seems to be not attending to this "bug" of loading related models.