I believe there is an issue here related to associations. By the time the "request_data" for an association reaches this point the root has been stripped out. This causes `request[:data]` to be an array with associated objects not a hash with the root.

For example:

```ruby
class Foo::PhoneNumber
  include Her::Model
  parse_root_in_json true, :format => :active_model_serializers
end

class Foo::User
  include Her::Model
  parse_root_in_json true, :format => :active_model_serializers

  has_many :phone_numbers
end
```

And we return data in the [default format for embedded associations](https://github.com/rails-api/active_model_serializers#embedding-associations):

```json
{
  "user": {
    "id": 1, 
    "fullname": "Lindsay Fünke",
    "phone_numbers": [
      { "id": 1, "digits": "8675309" }
    ]
  }
}
```

We'll receive the error: "no implicit conversion of Symbol into Integer".

I would submit a pull request but I'm not sure where the best place to resolve this issue is. I'm not sure if this method is being used to extract arrays that are not associations (ie. a request to /phone_numbers). To make things work locally I patched this method to add a check that we're actually dealing with a hash (something that could have a root element) like this:

```ruby
def extract_array(request_data)
  if active_model_serializers_format? && request_data[:data].is_a?(Hash)
    request_data[:data][pluralized_parsed_root_element]
  else
    request_data[:data]
  end
end
```