Fix checkstyle issue

Author: patrickfav

src/main/java/at/favre/lib/bytes/MutableBytes.java

@@ -178,6 +178,11 @@ public Bytes immutable() {
         return Bytes.wrap(internalArray(), byteOrder());
     }
 
+    @Override
+    public int hashCode() {
+        return Util.Obj.hashCode(internalArray(), byteOrder());
+    }
+
     @Override
     public boolean equals(Object o) {
         return super.equals(o);