Add testcase for internal used Comparator in ClientCookieEncoder (#9897)

Motivation:

https://github.com/netty/netty/pull/9883 added a bug-fix for the Comparator in ClientCookieEncoder but did not add a testcase.

Modifications:

- Add testcase
- Simplify code

Result:

Include a test to ensure we not regress.

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

@@ -91,7 +91,8 @@ public final class ClientCookieEncoder extends CookieEncoder {
      * Sort cookies into decreasing order of path length, breaking ties by sorting into increasing chronological
      * order of creation time, as recommended by RFC 6265.
      */
-    private static final Comparator<Cookie> COOKIE_COMPARATOR = new Comparator<Cookie>() {
+    // package-private for testing only.
+    static final Comparator<Cookie> COOKIE_COMPARATOR = new Comparator<Cookie>() {
         @Override
         public int compare(Cookie c1, Cookie c2) {
             String path1 = c1.path();
@@ -103,13 +104,10 @@ public final class ClientCookieEncoder extends CookieEncoder {
             // limited use.
             int len1 = path1 == null ? Integer.MAX_VALUE : path1.length();
             int len2 = path2 == null ? Integer.MAX_VALUE : path2.length();
-            int diff = len2 - len1;
-            if (diff != 0) {
-                return diff;
-            }
+
             // Rely on Arrays.sort's stability to retain creation order in cases where
             // cookies have same path length.
-            return 0;
+            return len2 - len1;
         }
     };
 

codec-http/src/test/java/io/netty/handler/codec/http/cookie/ClientCookieEncoderTest.java

@@ -51,4 +51,16 @@ public class ClientCookieEncoderTest {
     public void testRejectCookieValueWithSemicolon() {
         ClientCookieEncoder.STRICT.encode(new DefaultCookie("myCookie", "foo;bar"));
     }
+
+    @Test
+    public void testComparatorForSamePathLength() {
+        Cookie cookie = new DefaultCookie("test", "value");
+        cookie.setPath("1");
+
+        Cookie cookie2 = new DefaultCookie("test", "value");
+        cookie2.setPath("2");
+
+        assertEquals(0, ClientCookieEncoder.COOKIE_COMPARATOR.compare(cookie, cookie2));
+        assertEquals(0, ClientCookieEncoder.COOKIE_COMPARATOR.compare(cookie2, cookie));
+    }
 }