[#593] Updated getting `MyModel.document_type` in Tire::Model::Naming

ActiveModel 4 changed the object returned from `Model.model_name`,
so let's call `to_s` on it explicitely.

Related: #593
[#593] Updated calling model callbacks in `MyModel.update_index`

Use the `run_callbacks` method, not `_run_update_elasticsearch_index_callbacks`.

See commit: rails/rails@19357a7b023fd56a5b381cd8894bb520c60cdb59

Authored by @hellvinz and @romansklenar.

Closes #593, closes #585
Use correct invocation of `include_root_in_json` in the `to_indexed_json` example

The previous example attempted to disable JSON roots but
didn't reference "self" so it only assigned a variable.

Closes #581