The schema2 manifest builder fills in this part of the manifest based on
the descriptor it gets back from BlobIngester's Put method. It passes
the correct media type to Put, but Put ends up replacing this value with
application/octet-stream in its return value.

This commit works around the issue in the manifest builder. Arguably Put
should not be changing the media type in its return value, but this
commit is a targeted fix to keep it very low-risk for possible inclusion
in Docker 1.11.

Fixes #1621 (but maybe we should open a separate issue for the media
type behavior in the distribution client, and the unnecessary stat).

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>