`dockerPusher` provides `pushWriter` which implements `content.Writer`.
However, even if `pushWriter` become abort status (i.e. `Close()` is called
before `Commit()`), `dockerPusher` doesn't recognise that status and treats that
writer as on-going.
This behaviour doesn't allow the client to retry an aborted push.
This commit fixes this issue.
This commit also adds an test to ensure that the issue is fixed.
Signed-off-by: Kohei Tokunaga <ktokunaga.mail@gmail.com>
