Accept Block For Get Requests

Most storage providers accept a block for chunked downloads. Fog's Atmos implementation does not support this functionality hence the download_from_blobstore method (https://github.com/cloudfoundry/cloud_controller_ng/blob/master/lib/cloud_controller/blobstore/client.rb) fails. The method always results in a zero byte file downloaded.

This PR allows a block to be accepted and downloads to continue. 
I have not been able to add tests for this as there is no mocking structure in the Atmos implementation.