Add assert to RandomContentFile, ensure size >= digest_size
Add assert to RandomContentFile, ensure size >= digest_size

It clearly will not work to have the size smaller than the digest
size, so we shouldn't allow it.

dreamhost/s3-tests,smithfarm/s3-tests,Nexenta/s3-tests,andrewgaul/s3-tests,ceph/s3-tests,Nexenta/s3-tests,basho/s3-tests,Intel-bigdata/s3-tests,ceph/s3-tests,robbat2/s3-tests,dreamhost/s3-tests,smithfarm/s3-tests,robbat2/s3-tests,Nexenta/s3-tests,basho/s3-tests,Intel-bigdata/s3-tests,jmunhoz/s3-tests,SUSE/s3-tests,SUSE/s3-tests,timuralp/s3-tests,jmunhoz/s3-tests,tobegit3hub/s3-tests,tobegit3hub/s3-tests,piousbox/s3-tests,Nexenta/s3-tests,timuralp/s3-tests,piousbox/s3-tests,piousbox/s3-tests,andrewgaul/s3-tests