Document Expiration date added to Document Details

Document details endpoint and Webhooks now return document's expiration date.

API example:

{
    "id": "g2wE9fJikvJTPaeyYDe82J",
    ...
    "expiration_date": "2028-06-16T09:27:01.219415Z",
    "metadata": {},
    "tokens": [],
    ...
}

Webhook example:

[
  {
    "event": "document_state_changed",
    "data": {
      "id": "g2wE9fJikvJTPaeyYDe82J",
      "name": "Integration test",
      "date_created": "2018-08-08T11:39:56.871553Z",
      "date_modified": "2018-08-08T11:39:57.180209Z",
      "expiration_date": "2028-06-16T09:27:01.219415Z",
      ...
    }
  }
]