AsciiString contentEqualsIgnoreCase fails when arrayOffset is non-zero (#9477)

Motivation:

AsciiString.contentEqualsIgnoreCase may return true for non-matching strings of equal length when offset is non zero.

Modifications:

- Correctly take offset into account
- Add unit test

Result: 

Fixes #9475

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

@@ -532,7 +532,7 @@ public final class AsciiString implements CharSequence, Comparable<CharSequence>
 
         if (string instanceof AsciiString) {
             AsciiString rhs = (AsciiString) string;
-            for (int i = arrayOffset(), j = rhs.arrayOffset(); i < length(); ++i, ++j) {
+            for (int i = arrayOffset(), j = rhs.arrayOffset(), end = i + length(); i < end; ++i, ++j) {
                 if (!equalsIgnoreCase(value[i], rhs.value[j])) {
                     return false;
                 }
@@ -540,7 +540,7 @@ public final class AsciiString implements CharSequence, Comparable<CharSequence>
             return true;
         }
 
-        for (int i = arrayOffset(), j = 0; i < length(); ++i, ++j) {
+        for (int i = arrayOffset(), j = 0, end = length(); j < end; ++i, ++j) {
             if (!equalsIgnoreCase(b2c(value[i]), string.charAt(j))) {
                 return false;
             }

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

@@ -46,6 +46,15 @@ public class AsciiStringCharacterTest {
     private static final Charset[] CHARSETS = new Charset[]
             { UTF_16, UTF_16BE, UTF_16LE, UTF_8, ISO_8859_1, US_ASCII };
 
+    @Test
+    public void testContentEqualsIgnoreCase() {
+        byte[] bytes = { 32, 'a' };
+        AsciiString asciiString = new AsciiString(bytes, 1, 1, false);
+        // https://github.com/netty/netty/issues/9475
+        assertFalse(asciiString.contentEqualsIgnoreCase("b"));
+        assertFalse(asciiString.contentEqualsIgnoreCase(AsciiString.of("b")));
+    }
+
     @Test
     public void testGetBytesStringBuilder() {
         final StringBuilder b = new StringBuilder();