Use path.Join in proxy_server in pkg kubectl

There is a `singleJoiningSlash` func that used to handle join("http://host:port", "/hello"). 
But we could parse the url first, then join the url.Path with "/hello". 
So we can get rid of `singleJoiningSlash` and use path.Join.
Ping on this PR-- I would like to merge it if you can add a test that verifies this parsing step is right. Thanks!