Fixed, that models with custom `as_indexed_json` method are properly serialized during update

Previously, the `update_document` method simply intercepted the changes to the model, via the
`@__changed_attributes` variable, and used these directly.

This caused models with a custom serialization method to be incorrectly serialized, namely
unwanted attributes were added.

This patch looks for `as_indexed_json` defined on the model, and when it finds it,
filters the changed attributes through the keys.

Closes #75

Related:

* #59
* #57
* #52
* #40
* #37
* #5