Add utf8_encode to messageBody for utf8 conversion.

I was having trouble with fetching messages due to an illegal character when it was running the iconv() function.

```
[Symfony\Component\Debug\Exception\ContextErrorException]                                                                                  
  Notice: iconv(): Detected an illegal character in input string in /var/www/sites/cems2/vendor/tedivm/fetch/src/Fetch/Message.php line 430  
```

Since the purpose of using iconv() in this circumstance appears to be to convert it from its existing character set to utf-8, encoding the messageBody with utf8_encode() seemed appropriate. All tests and builds passed for me locally.