Handle unstructured field with UTF-7 encoded-word

Attempting to parse an email with the following header:

```
References: =?utf-7?B?PDEyMzRAdGVzdC5saW5kc2Fhci5uZXQ+?=
```

Will cause an error: `Encoding::ConverterNotFoundError: code converter not found (UTF-7 to UTF-8)`.

This can be fixed by checking for UTF-7 within the `b_value_decode` method and using Net::IMAP to decode the string first.