Unnecessary encoding

```javascript
import rfc2047 from 'rfc2047'
console.log(rfc2047.encode("uvwxy-Bucureşti <uvwxy-bucuresti@uvwxy.eu>"))
```
prints `=?utf-8?Q?uvwxy-Bu?= =?utf-8?Q?cure=C5=9Fti?= <uvwxy-bucuresti@uvwxy.eu>`.

The problem report is, that there is no need to encode `uvwxy-Bu` as `=?utf-8?Q?uvwxy-Bu?=`.