Copy request body in ProxyServer (kubecfg -proxy)

Fixes #1149 - kubecfg proxy "411 Length Required" error on POST/PUT.
Yeah sorry I didn't see at first the bug you were trying to fix. See my previous comment-- would prefer to fix this by reusing other proxy code.

Barring that, there has to be a better fix than reading everything-- that spoils the purpose of passing around io.Reader. It's puzzling that this actually changes the behavior. I suspect there's a header or something that needs to be set differently in client's doRequest. But regardless, would rather fix by deleting this function entirely :)