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_test.go

@@ -7,6 +7,8 @@ import (
 	"net/http/httptest"
 	"reflect"
 	"testing"
+
+	"github.com/mholt/caddy"
 )
 
 func TestParseClientHello(t *testing.T) {
@@ -285,7 +287,7 @@ func TestHeuristicFunctionsAndHandler(t *testing.T) {
 			want := ch.interception
 			handler := &tlsHandler{
 				next: http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-					got, checked = r.Context().Value(CtxKey("mitm")).(bool)
+					got, checked = r.Context().Value(caddy.CtxKey("mitm")).(bool)
 				}),
 				listener: newTLSListener(nil, nil),
 			}