This fix is an attempt to fix issue raised in #28005 where
`docker stats` on Windows shows Linux headers if there is
no containers in stats.
The reason for the issue is that, in case there is no container,
a header is faked in:
https://github.com/docker/docker/blob/v1.13.0/cli/command/formatter/formatter.go#L74-L78
which does not know OS type information (as OS was stored with container stat entries)
This fix tries to fix the issue by moving OS type information
to stats context (instead of individual container stats entry).
Additional unit tests have been added.
This fix fixes#28005.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>