Allow sending files with any status

Fixes #3037. Seems like it was unintentional that `Status#sendFile` only works with `Ok`.

@guillaumebort @jroper
I would use the specs2 `and` combinator:

```scala
(result.header.status aka "status" must_== OK).
  and(result.header.headers.get(CONTENT_DISPOSITION) aka "disposition" must beSome(""""attachment; filename="test.tmp"""")
```