Add epoch_ms to dynamic dates with format yyyy/MM/dd

[clintongormley]

When creating date fields dynamically:

PUT my_index/my_type/1
{
  "date_one": "2015-01-01", 
  "date_two": "2015/01/01"  
}

the date matching strict_date_optional_time adds the ||epoch_millis format as an alternative, but the date matching yyyy/MM/ss doesn't:

{
   "my_index": {
      "mappings": {
         "my_type": {
            "properties": {
               "date_one": {
                  "type": "date",
                  "format": "strict_date_optional_time||epoch_millis"
               },
               "date_two": {
                  "type": "date",
                  "format": "yyyy/MM/dd HH:mm:ss||yyyy/MM/dd"
               }
            }
         }
      }
   }
}

I think it should.