Make keepalive setting conditional

[gagara]

Keepalive option at https://github.com/logstash-plugins/logstash-input-http/blob/master/src/main/java/org/logstash/plugins/inputs/http/NettyHttpServer.java#L57 should not be hardcoded, but it must respect "Connection" header.
If in plugin configuration user provides custom response header as

response_headers => {"Connection" => "close"}

then in according to https://tools.ietf.org/html/rfc7230#section-6.6 server must close connection

A server that sends a "close" connection option MUST initiate a close
of the connection (see below) after it sends the response containing
"close". The server MUST NOT process any further requests received
on that connection.