pkg/tarsum: adding more tests

Ensuring case size of headers will still be accounted for.
https://github.com/docker/docker/pull/8869#discussion_r20114401

Signed-off-by: Vincent Batts <vbatts@redhat.com>
Merge pull request #9086 from vbatts/vbatts-tarsum_more_tests

pkg/tarsum: adding more tests