Remove _id if not present
Remove _id if not present

## Description
Elasticsearch 5.x complains (gives an error) if we pass an empty `_id`, so if the input doesn't have a populated `_id` field the import will fail.
If we remove this field from the import header, elasticsearch will auto-generate it normally!

## How to test

1- start an elasticsearch instance.
This docker command can help:
```
docker run -it --rm -p 9200:9200 -p 9300:9300 -p 5601:5601  -e ES_HEAP_SIZE="256m" -e ES_JAVA_OPTS="-Xms256m -Xmx256m" -e LOGSTASH_START=0 --name elk --security-opt seccomp=unconfined sebp/elk
```

2- run `elastic-import` with a csv file without the `_id` field
```
elastic-import example.csv localhost:9200 example type --csv -p --header-fields
```
rubenjgarcia/elastic-import