[CLIENT] Support compression when using Manticore adapter with JRuby

Author: estolfo

elasticsearch-transport/lib/elasticsearch/transport/transport/http/manticore.rb

@@ -158,6 +158,7 @@ def apply_headers(request_options, options)
             headers = (options && options[:headers]) || {}
             headers[CONTENT_TYPE_STR] = find_value(headers, CONTENT_TYPE_REGEX) || DEFAULT_CONTENT_TYPE
             headers[USER_AGENT_STR] = find_value(headers, USER_AGENT_REGEX) || user_agent_header
+            headers[ACCEPT_ENCODING] = GZIP
             request_options.merge!(headers: headers)
           end