Bulk create throws invalid BulkOperationException

When using:
```python
es.index(doc, ELASTICSEARCH_INDEX, document_type, bulk=True,  force_insert=True)
es.flush_bulk(forced=True)
```
This was throwing a invalid BulkOperationException exception.
Merge pull request #343 from boekkooi/master

Bulk create throws invalid BulkOperationException