ble`within` does not seem to respect `as`

From the readme, `within`/`as`/`with` should be stackable. But this scenario, that does not work as expected:

``` ruby
require 'sshkit'
require 'sshkit/dsl'

include SSHKit::DSL

# Set up a directory that your user cannot enter
`sudo mkdir -p /tmp/only_root`
`sudo chmod og-rwx /tmp/only_root`

on :local do
# But root should be able to enter it...
as :root do
within '/tmp/only_root' do
execute :ls # --> Permission denied
end
end
end
```

It looks like `within` always runs as the user executing the script, which is unexpected. I would have thought that it's done as the user of the enclosing `as`. I've tried with the local and netssh backends and get the same results.

This is with SSHKit 1.11.3 and Ruby 2.3.1 on Centos 6.