Use RequestURI when redirecting to canonical path. (#1331)

* Use RequestURI when redirecting to canonical path.

Caddy may trim a request's URL path when it starts with the path that's
associated with the virtual host. This change uses the path from the request's
RequestURI when performing a redirect.

Fix issue #1327.

* Rename redirurl to redirURL.

* Redirect to the full URL.

The scheme and host from the virtual host's site configuration is used
in order to redirect to the full URL.

* Add comment and remove redundant check.

* Store the original URL path in request context.

By storing the original URL path as a value in the request context,
middlewares can access both it and the sanitized path. The default
default FileServer handler will use the original URL on redirects.

* Replace contextKey type with CtxKey.

In addition to moving the CtxKey definition to the caddy package, this
change updates the CtxKey references in the httpserver, fastcgi, and
basicauth packages.

* httpserver: Fix reference to CtxKey

caddyhttp/httpserver/mitm.go

@@ -9,6 +9,8 @@ import (
 	"net/http"
 	"strings"
 	"sync"
+
+	"github.com/mholt/caddy"
 )
 
 // tlsHandler is a http.Handler that will inject a value
@@ -72,7 +74,7 @@ func (h *tlsHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	}
 
 	if checked {
-		r = r.WithContext(context.WithValue(r.Context(), CtxKey("mitm"), mitm))
+		r = r.WithContext(context.WithValue(r.Context(), caddy.CtxKey("mitm"), mitm))
 	}
 
 	if mitm && h.closeOnMITM {