Use correct default value for the overwrite_existing parameter.
Use correct default value for the overwrite_existing parameter.

See https://github.com/rhec/pyelasticsearch/commit/88950a96583e538f9d38f4e356a995819a4739ac#L2L288
einvalentin/elasticutils,mozilla/elasticutils,einvalentin/elasticutils,einvalentin/elasticutils,mozilla/elasticutils,mozilla/elasticutils