subclasses of custom Model do not inherit custom ArrayModel

[alexandred]

This is easiest to demonstrate with the following example:

class StorageBase < Volt::Model
end

class StorageBases < Volt::ArrayModel
end

class Attachment < StorageBase
end

In this case, store.storage_bases is an instance of StorageBases but store.attachments is an instance of Volt::ArrayModel. The expected behaviour would be that store.attachments is an instance of StorageBases.

[ryanstout]

PR merged. Thanks