'line' not being 'group'ed

I'm not quite experienced enough with this library to confidently call this a bug, but the following behaviour certainly seems odd:

``` haskell
putDoc $ group $ "a" <> line <> "b" <> hardline <> "c"
```

outputs:

```
a
b
c
```

as though the use of `hardline` has made the `line` hard as well. The same result is seen with `"a" <> hardline <> "b" <> line <> "c"`.