Invalid UTF-8 value raises UnicodeDecodeError

Using quoting, a cookie value can have any byte sequence. When the sequence is invalid UTF-8, Cookie raises an UnicodeDecodeError.

This is an example of a bad cookie:
```
$ curl -b C=%80 http://webappusingcookie:8000
```

Depending of the web application implementation, that request can give a 500 error or raise a UnicodeDecodeError.

I suggest to use `.decode('utf-8-, 'replace')` in `parse_string` method. I also have added a test case.