Node.js v6 will go EOL at the end of April 2019, and already two of the production dependencies of the client have already dropped support for it, and soon others will do *(as well as development dependencies)*.
Furthermore, since Node.js will go in EOL it will never get security patches, plus, also OpenSSL-1.0.2 will go EOL [this year](https://github.com/nodejs/Release#release-schedule); to avoid risks for the client users it is better to drop support for Node.js v6 right away.
In very few cases, some API uses the same key for both url and query params, such as the bulk method.
The client is not designed to handle such cases since accepts both url and query keys in the same object, and the url parameter will always take precedence.
This pr fixes this edge case by adding a `querystring` key in the options object.
Fixes: https://github.com/elastic/elasticsearch-js/pull/778
```js
client.bulk({
index: 'index',
type: '_doc',
body: [...]
}, {
querystring: {
type: '_doc'
}
}, console.log)
```
