Refactor: extract ETag generation from manifests
Remove maybe_ prefix from test function name
Fix Etag generation from source manifests

If source was uploaded by multipart, manifest's content-md5 is not
binary but 2-tuple.
Add test of PUT Copy from MP uploaded object to normal object
Fix unexpected 503 error when monifest does not exist

Expected status code is 404
Fix etag generation which should NOT be quoted
Merge pull request #901 from basho/bugfix/copy-from-multipart

Bugfix/copy from multipart

Reviewed-by: kuenishi