bug: Use of `iconv()` causing server 503!

This line in `Encoder::append8BitBytes()` appears to cause some error which results in a 503 from the server.

```php
$bytes = @iconv('utf-8', $encoding, $content);
```

replacing it with

```php
$bytes = mb_convert_encoding($content, $encoding, 'UTF-8');
```

fixes the issue for me.

I see now that this relates to PR #178 .