elasticsearch: upgrade from version 2 to 6

> Content-Type header [application/x-www-form-urlencoded] is not supported

https://www.elastic.co/blog/strict-content-type-checking-for-elasticsearch-rest-requests

No handler for type [string] declared on field [h1]

https://stackoverflow.com/questions/47452770/no-handler-for-type-string-declared-on-field-name

Could not convert [language.index] to boolean

https://www.elastic.co/guide/en/elasticsearch/reference/5.0/breaking_50_mapping_changes.html#breaking_50_mapping_changes https://www.elastic.co/blog/strings-are-dead-long-live-strings

pre_tags are set but post_tags are not set

Fielddata is disabled on text fields by default. Set fielddata=true on [tags] in order to load fielddata in memory by uninverting the inverted index. Note that this can however use significant memory. Alternatively use a keyword field instead.