make Elasticsearch.scroll POST the scroll ID

Without this, if you have a large number of indices the `scroll_id` will be very long, and ElasticSearch will return an invalid response. (A urllib3 `ConnectionError` to be precise.) This changes the behavior to POST to the scroll endpoint, which gets around the URL length restriction.

This is an implementation of the solution to the same problem [in the elasticsearch-users](http://elasticsearch-users.115913.n3.nabble.com/Ridiculously-long-Scroll-id-td4038567.html) mailing list.

I'm just opening this to see if it's a welcome change, if need be I can change whatever tests need to be fiddled with as well.