Copy request body in ProxyServer (kubecfg -proxy)

Fixes #1149 - kubecfg proxy "411 Length Required" error on POST/PUT.
I'll try to reuse apiserver's proxy code and see if it fixes the bug. 

When passing io.Reader directly the proxy req header is set to chunked correctly but body bytes are not always present. Really weird bug. 

Plus, Nginx always complains about missing content-length.

Anyway, will report back here when I replace this with the other proxy code.