client: Fix failed to pull ubuntu image from docker.io

[arronwy]

Manifests list feature in oci-distribution is already implemented, default image in docker.io like busybox use IMAGE_MANIFEST_LIST_MEDIA_TYPE, but ubuntu image use OCI_IMAGE_INDEX_MEDIA_TYPE.

Fixes: #27

[flavio]

@arronwy Given I don't know how much time it will take for distribution to fix this issue, and given how easy the fix is on our side I propose the following approach:

• We create an issue to keep track of the missing unit test
• We merge this fix without any unit test

What do you think about that?

[arronwy]

@arronwy Given I don't know how much time it will take for distribution to fix this issue, and given how easy the fix is on our side I propose the following approach:

• We create an issue to keep track of the missing unit test
• We merge this fix without any unit test

What do you think about that?

Agree, thanks a lot!

[arronwy]

I have updated this PR, and create an issue to track uni tests: https://github.com/krustlet/oci-distribution/issues/68