Fix data race in MITM'ed HTTPS request handling.[comment](golang/go#6…

…1596 (comment))

[fixed] use the same context

[add] websocket proxy example

[fixed] examples/mitm-proxy

You have to reset Content-Length, if you change the Body

Socks5 will be implemented in another library, including more underly…

…ing protocols such as TCP, UDP, KCP