Fix parsing of proto/port - issue 6447

Fixes #6447
Here's what I think is going on.  Using the old code at: https://github.com/docker/docker/blob/master/nat/nat.go#L65   I think the parsing is a bit off.  The old parse would do this (  "old string" -> port / protocol ):
123/tcp -> 123/tcp
123/ -> 123/""    (#6447 issue)
/tcp -> tcp/""    (error)
/ -> ""/""
"" -> ""/""

And I think its the 123/ -> 123/"" case that is the cause of the issue in #6447, it should return 123/tcp.
With this change the parser now returns:
123/tcp -> tcp/123
123/  ->  123/tcp  (now it'll default to tcp to fix #6447)
/tcp  -> ""/""   (treat the same as / and "")
/  -> ""/""
"" ->  ""/""

I also added some new tests to enforce this.

Signed-off-by: Doug Davis <dug@us.ibm.com>