Bug: upload! within an `as 'root'` block will fail.

The following will usually fail:

```
task :foo do
on roles(:web) do
as "root" do
upload! "/tmp/foo", "/var/bar/cuux"
end
end
end
```

Since scp will not su to root. Would you accept a PR that allows `upload!` to upload to `/tmp` and then `mv` the file into place, if the current user has been switched?