Added support for sending data with delete requests

Unusual, but needed by elasticsearch for the delete by query API. 

(see http://www.elasticsearch.org/guide/reference/api/delete-by-query/)
The only reason is that I wanted to provide a minimum patch that works and be as less invasive as possible, trying also to respect the same policy used in other methods in the same library (e.g. #get). So if we have a broader request method that now supports also delete with data, we can use that one without making any other API change.