Remove unnecessary loop variable from AsciiString. (netty#8711)

Motivation:

Incrementing two variables in sync is not necessary when only one will do.

Modifications:

- Remove `j` from `for` loop and replace with `i`.
- Add more unit testing scenarios to cover changed code.

Results:

Unnecessary variable removed.

common/src/main/java/io/netty/util/AsciiString.java

@@ -1452,8 +1452,8 @@ public static boolean contentEqualsIgnoreCase(CharSequence a, CharSequence b) {
         if (a.length() != b.length()) {
             return false;
         }
-        for (int i = 0, j = 0; i < a.length(); ++i, ++j) {
-            if (!equalsIgnoreCase(a.charAt(i),  b.charAt(j))) {
+        for (int i = 0; i < a.length(); ++i) {
+            if (!equalsIgnoreCase(a.charAt(i),  b.charAt(i))) {
                 return false;
             }
         }

common/src/test/java/io/netty/util/AsciiStringCharacterTest.java

@@ -240,6 +240,9 @@ public void testEqualsIgnoreCase() {
         assertThat(AsciiString.contentEqualsIgnoreCase(null, "foo"), is(false));
         assertThat(AsciiString.contentEqualsIgnoreCase("bar", null), is(false));
         assertThat(AsciiString.contentEqualsIgnoreCase("FoO", "fOo"), is(true));
+        assertThat(AsciiString.contentEqualsIgnoreCase("FoO", "bar"), is(false));
+        assertThat(AsciiString.contentEqualsIgnoreCase("Foo", "foobar"), is(false));
+        assertThat(AsciiString.contentEqualsIgnoreCase("foobar", "Foo"), is(false));
 
         // Test variations (Ascii + String, Ascii + Ascii, String + Ascii)
         assertThat(AsciiString.contentEqualsIgnoreCase(new AsciiString("FoO"), "fOo"), is(true));