net/http: allow ListenAndServeTLS to omit PEM file values if TLSConfig.GetConfigForClient set

[TheTesla]

If I set GetConfigForClient and start the server using ListenAndServeTLS the arguments of ListenAndServeTLS, the server certificate and its key are lneeded to be given and they are also loaded. I have set the server certificate and its key in the configuration TLSConfig served by GetConfigForClient before. So using GetConfigForClient ListenAndServeTLS should ignore its arguments and they should also be optional.

[gopherbot]

Change https://golang.org/cl/175457 mentions this issue: GetConfigForClient replaces cert and key arguments

[mikedanese]

What config should be used if the client doesn't send a hello?

[seankhliao]

Done in CL 578396