Switch backup to return a base64 encoded sha1 hash instead of a hex-encode sha 256
Adding a comment
Switch to Digest header for sending download hash
Merge pull request #130 from voidspace/backup-header

Switch backup download to Digest header

apiserver/backup: switch the http header to the RFC 3230 Digest header

The backup download api sends an sha hash as a header. RFC 3230
defines a standard for this header, using a base64 encoded SHA1 hash.