Make Registry support recursive bindData

This is just a little recommend of making Registry recursive merge a new loading file.

### Before

``` php
$json1 = '{
    "field" : {
        "keyA" : "valueA",
        "keyB" : "valueB"
    }
}';

$json2 = '{
    "field" : {
        "keyB" : "a new valueB"
    }
}';

$registry->loadString($json1);
$registry->loadString($json2);
```

Output

```
Array(
    field => Array(
        keyB => a new valueB
    )
)
```

### After

``` php
$json1 = '{
    "field" : {
        "keyA" : "valueA",
        "keyB" : "valueB"
    }
}';

$json2 = '{
    "field" : {
        "keyB" : "a new valueB"
    }
}';

$registry->loadString($json1);
$registry->loadString($json2);
```

Output

```
Array(
    field => Array(
        keyA => valueA
        keyB => a new valueB
    )
)
```