MIME4J-305 ContentUtil::decode can avoid using StringBuilder (#51)

StringBuilder is an expensive construct, we spend most of our time, for each character, ensuring the capacity of the string builder.

Size being known, the operation simple, we can directly operate on top of a char array.

Gains: 78%

core/src/main/java/org/apache/james/mime4j/util/ContentUtil.java

@@ -176,11 +176,11 @@ public static String decode(ByteSequence byteSequence, int offset, int length) {
         if (byteSequence == null) {
             return null;
         }
-        StringBuilder buf = new StringBuilder(length);
+        char[] underlying = new char[length];
         for (int i = offset; i < offset + length; i++) {
-            buf.append((char) (byteSequence.byteAt(i) & 0xff));
+            underlying[i - offset] = (char) (byteSequence.byteAt(i) & 0xff);
         }
-        return buf.toString();
+        return new String(underlying);
     }
 
     /**