Avoid possible comparison contract violation (netty#9883)

Motivation:

The current implementation causes IllegalArgumetExceptions to be thrown on Java 11.

The current implementation would violate comparison contract for two cookies C1 and C2 with same path length, since C1 < C2 and C2 < C1. Returning 0 (equality) does not since C1 == C2 and C2 == C1. See netty#9881

Modification:

Return equality instead of less than on same path length.

Result:

Fixes netty#9881.

codec-http/src/main/java/io/netty/handler/codec/http/cookie/ClientCookieEncoder.java

@@ -107,9 +107,9 @@ public int compare(Cookie c1, Cookie c2) {
             if (diff != 0) {
                 return diff;
             }
-            // Rely on Java's sort stability to retain creation order in cases where
+            // Rely on Arrays.sort's stability to retain creation order in cases where
             // cookies have same path length.
-            return -1;
+            return 0;
         }
     };