Elasticsearch: Add charset

[pecorarista]

Purpose

Queries sent from ElasticsearchSource.typed ignore non-ascii characters.
For example, the query created by the following code

ElasticsearchSource
  .typed[Document](
    "documents",
    "_doc",
    query = """{ "match": { "content": "先生" } }"""
  )

is recorded in logs/elasticsearch_index_search_slowlog.log as

source[{"size":10,"query":{"match":{"content":{"query":"??","operator":"OR","prefix_length":0,"max_expansions":50,"fuzzy_transpositions":true,"lenient":false,"zero_terms_query":"NONE","auto_generate_synonyms_phrase_query":true,"boost":1.0}}},"sort":[{"_doc":{"order":"asc"}}]}]

where "先生" is "??".

Changes

• Add StandardCharset.UTF_8 to StringEntity in ElasticsearchSourceStage.scala as in ElasticsearchFlowStage.scala

[lightbend-cla-validator]

Hi @pecorarista,

Thank you for your contribution! We really value the time you've taken to put this together.

Before we proceed with reviewing this pull request, please sign the Lightbend Contributors License Agreement:

http://www.lightbend.com/contribute/cla

[ennru]

LGTM.

[ennru]

Thank you!