Remove default type as Elastic 7 deprecates document types

[deric]

In order to be compatible with recent Elastic versions document type information should not be defined or _doc type could be used.

_doc is a dummy document type that will work as well with older Elastic versions

See: https://www.elastic.co/guide/en/elasticsearch/reference/7.x/removal-of-types.html

[trink]

Changing the defaults is a breaking change for anyone using pre v7. Ideally the encoder would accept a version and then apply the correct defaults (the version should default to 5 (when nil) since that is what this encoder was originally created against).

Also the elasticsearch package version needs to be incremented 1.0.9
https://github.com/mozilla-services/lua_sandbox_extensions/blob/master/elasticsearch/CMakeLists.txt#L6

[deric]

@trink You're right, it would be a breaking change for anyone relying on default values. I wasn't sure how to get the Elastic version, alternatively we could send GET elasticsearch:9200 and parse the version... but then the type would be determined upon connection to Elastic.