Fix loading of nested attributes (Hashes) in REST adapter

This PR fixes loading of hashes from JSON. The previous way caused key-value attributes to be loaded as JS `Object` into opal, instead of `Hash`.

Also `Model#from_form` was updated to normalise hash keys and values:

```ruby
'user[address]' => 'value'
#  becomes:
'user' => { 'address' => 'value' }

'user[]' => 'value'
# becomes:
'user' => [ 'value' ]
```

Another thing this PR fixes is `Persistence#did_destroy`, which weren't removing items from the RecordArray.