More helpful error when allowed redirect domains are defined (#104)

Make the error message when a forbidden destination is given to
`/redirect-to`, to help users better understand what allowed usage looks
like. Motivated by #103.

Before: 

```
$ curl http://localhost:8080/redirect-to?url=https://google.com
Forbidden redirect URL. Be careful with this link.
```

After:

```
$ curl http://localhost:8080/redirect-to?url=https://google.com
Forbidden redirect URL. Please be careful with this link.

Allowed redirect destinations:
- example.com
- example.org
```

httpbin/handlers.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"net/http"
 	"net/url"
+	"sort"
 	"strconv"
 	"strings"
 	"time"
@@ -390,7 +391,17 @@ func (h *HTTPBin) RedirectTo(w http.ResponseWriter, r *http.Request) {
 
 	if u.IsAbs() && len(h.AllowedRedirectDomains) > 0 {
 		if _, ok := h.AllowedRedirectDomains[u.Hostname()]; !ok {
-			http.Error(w, "Forbidden redirect URL. Be careful with this link.", http.StatusForbidden)
+			domainListItems := make([]string, 0, len(h.AllowedRedirectDomains))
+			for domain := range h.AllowedRedirectDomains {
+				domainListItems = append(domainListItems, fmt.Sprintf("- %s", domain))
+			}
+			sort.Strings(domainListItems)
+			formattedDomains := strings.Join(domainListItems, "\n")
+			msg := fmt.Sprintf(`Forbidden redirect URL. Please be careful with this link.
+
+Allowed redirect destinations:
+%s`, formattedDomains)
+			http.Error(w, msg, http.StatusForbidden)
 			return
 		}
 	}

httpbin/handlers_test.go

@@ -74,7 +74,7 @@ func assertBodyEquals(t *testing.T, w *httptest.ResponseRecorder, want string) {
 	t.Helper()
 	have := w.Body.String()
 	if want != have {
-		t.Fatalf("expected body = %v, got %v", want, have)
+		t.Fatalf("expected body = %q, got %q", want, have)
 	}
 }
 
@@ -1239,6 +1239,13 @@ func TestRedirectTo(t *testing.T) {
 		WithObserver(StdLogObserver(log.New(io.Discard, "", 0))),
 	).Handler()
 
+	allowedDomainsError := `Forbidden redirect URL. Please be careful with this link.
+
+Allowed redirect destinations:
+- example.org
+- httpbingo.org
+`
+
 	allowListTests := []struct {
 		url            string
 		expectedStatus int
@@ -1257,6 +1264,9 @@ func TestRedirectTo(t *testing.T) {
 			w := httptest.NewRecorder()
 			allowListHandler.ServeHTTP(w, r)
 			assertStatusCode(t, w, test.expectedStatus)
+			if test.expectedStatus >= 400 {
+				assertBodyEquals(t, w, allowedDomainsError)
+			}
 		})
 	}
 }