Improve [proxy support]: correct work when ES is on behind proxy (nginx)

When Elasticsearch is placed on behind the nginx server, then nginx return 411 error, because Content-Length header is required. Similar problem: https://github.com/elasticsearch/elasticsearch-py/issues/70

Example.
ES url http://domain.com/elastic-proxy/
Nginx config:
```
location /elastic-proxy/ {
       proxy_pass http://localhost:9200/;
}
```

Аdditionally _path_ parameter has been added, because ES behind proxy may be accessible not by "/" url, but with custom preffix.

Thx for this backend, very helpful.