8320577: Improve MessageHeader's toString() function to make HttpURLConnection's debug log readable

Backport-of: aac43184319d852eb792c83dfb52d74a3126108d

Author: cost0much

src/java.base/share/classes/sun/net/www/MessageHeader.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1995, 2021, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1995, 2023, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -609,7 +609,7 @@ public void mergeHeader(InputStream is) throws java.io.IOException {
     }
 
     public synchronized String toString() {
-        String result = super.toString() + nkeys + " pairs: ";
+        String result = super.toString() + " " + nkeys + " pairs: ";
         for (int i = 0; i < keys.length && i < nkeys; i++) {
             result += "{"+keys[i]+": "+values[i]+"}";
         }