[Ingest] default_pipeline is not working when the index is referred by its alias

[squirrel532]

Elasticsearch version : 6.5.0

OS version (uname -a if on a Unix-like system): Ubuntu 18.04

Description of the problem including expected versus actual behavior:

Steps to reproduce:

1. Create a pipeline that will modify _index field of the document.

PUT _ingest/pipeline/index_routing
{
    "description" : "route document to proper index and shard",
    "processors" : [
      {
        "date_index_name" : {
          "field" : "posted_date",
          "date_formats" : [
            "ISO8601"
          ],
          "date_rounding" : "y",
          "index_name_format" : "yyyy",
          "index_name_prefix" : "search-"
        }
      }
    ]
}

2. Create a index with default_pipeline

PUT test_index
{
  "settings": {
    "default_pipeline": "index_routing"
  },
  "aliases": {
    "my_index_alias": {}
  }
}

3. Create destination index

PUT search-2018
{}

4. Index something

PUT my_index_alias/_doc/first_doc
{
  "posted_date": "2018-01-01T00:00:00"
}

Then we can find that the document has been indexed into test_index instead of search-2018.

[elasticmachine]

Pinging @elastic/es-core-features