fix: use default title based on status code

Author: rluders

problem_details.go

@@ -1,6 +1,9 @@
 package httpsuite
 
-import "sync"
+import (
+	"net/http"
+	"sync"
+)
 
 const BlankUrl = "about:blank"
 
@@ -30,6 +33,12 @@ func NewProblemDetails(status int, problemType, title, detail string) *ProblemDe
 	if problemType == "" {
 		problemType = BlankUrl
 	}
+	if title == "" {
+		title = http.StatusText(status)
+		if title == "" {
+			title = "Unknown error"
+		}
+	}
 	return &ProblemDetails{
 		Type:   problemType,
 		Title:  title,