Server.ServeTLS wraps Server.Serve with added TLS support. This is
particularly useful for serving on manually initialized listeners.
Example use-case includes ability to serve with TLS on listener
provided by systemd's socket activation.
A matching test heavily based on TestAutomaticHTTP2_ListenAndServe
is also included.
Original code by Gurpartap Singh as
https://go-review.googlesource.com/c/38114/