in lib/elastic/transport/transport/base.rb [439:446]
def apply_headers(client, options)
headers = options[:headers].clone || {}
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(client)
client.headers[ACCEPT_ENCODING] = GZIP if use_compression?
client.headers.merge!(headers)
end