external_reference is missing task_id in data

[kspeer825]

Expected: When the time_entries table is updated, the external_reference table is also updated. It is expected that the external_reference stream is defined by the following JSON structure:

{
  "type": "object",
  "properties": {
    "id": {
      "type": ["null", "string"]
    },
    "task_id": {
      "type": ["null", "integer"]
    },
    "group_id": {
      "type": ["null", "string"]
    },
    "permalink": {
      "type": ["null", "string"]
    },
    "service": {
      "type": ["null", "string"]
    },
    "service_icon_url": {
      "type": ["null", "string"]
    }
  }
}

Actual: The tap is currently sending data for the external_reference without the task_id field.

To reproduce: Create a time entry with an external reference value that is not null using the Harvest API.