Move RedirectToHTTPS to middleware package

Moves the logic for redirecting to HTTPs to a middleware package and adds tests for this logic.
Also makes the functionality more useful, previously it always redirected to the HTTPS address of the proxy, which may not have been intended, now it will redirect based on if a port is provided in the URL (assume public facing 80 to 443 or 4180 to 8443 for example)

main.go

@@ -3,6 +3,7 @@ package main
 import (
 	"fmt"
 	"math/rand"
+	"net"
 	"os"
 	"os/signal"
 	"runtime"
@@ -79,7 +80,11 @@ func main() {
 	chain := alice.New()
 
 	if opts.ForceHTTPS {
-		chain = chain.Append(newRedirectToHTTPS(opts))
+		_, httpsPort, err := net.SplitHostPort(opts.HTTPSAddress)
+		if err != nil {
+			logger.Fatalf("FATAL: invalid HTTPS address %q: %v", opts.HTTPAddress, err)
+		}
+		chain = chain.Append(middleware.NewRedirectToHTTPS(httpsPort))
 	}
 
 	healthCheckPaths := []string{opts.PingPath}