Add equality check to contentEquals instance methods. (#9130)

Motivation:

An instance is always equal to itself. It makes sense to skip processing for this case, which isn't uncommon since `AsciiString` is often memoized within an application when used as HTTP header names.

Modification:

`contentEquals` methods first check for instance equality before doing processing.

Result:

`contentEquals` will be faster when comparing an instance with itself.

I couldn't find any unit tests for these methods, only the static version. Let me know if I should add something to `AsciiStringCharacterTest`.

Came up here:
https://github.com/line/armeria/pull/1731#discussion_r280396280

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

@@ -522,6 +522,10 @@ public final class AsciiString implements CharSequence, Comparable<CharSequence>
      * @return {@code true} if the specified string is equal to this string, {@code false} otherwise.
      */
     public boolean contentEqualsIgnoreCase(CharSequence string) {
+        if (this == string) {
+            return true;
+        }
+
         if (string == null || string.length() != length()) {
             return false;
         }
@@ -1036,6 +1040,10 @@ public final class AsciiString implements CharSequence, Comparable<CharSequence>
      * @return {@code true} if equal, otherwise {@code false}
      */
     public boolean contentEquals(CharSequence a) {
+        if (this == a) {
+            return true;
+        }
+
         if (a == null || a.length() != length()) {
             return false;
         }