Wrong puny_encode with hyphen 

Version: `urltools_1.7.3`

Actual:

```
urltools::puny_encode('foo-bar.tld')
[1] "xn--foo-bar-.tld"
```

Expected:

```
urltools::puny_encode('foo-bar.tld')
[1] "foo-bar.tld"
```

Reason: `foo-bar.tld` only contains LDH labels and thus should not be converted to `xn--` notation