Using `as` and sudo with NOPASSD and !requiretty

I'm having a lot of trouble using the `as` syntax. I realize this is more of a support request than a bug report, but the results of this might go into the documentation.

What doesn't work:

```
task :clear_cache do
on roles :app do
as :web1 do
within shared_path do
execute :rm, '-rf', 'src/cache/twig'
end
end
end
end
```

However, this works:

```
task :clear_cache do
on roles :app do
within shared_path do
execute :sudo, '-u', 'web1', :rm, '-rf', 'src/cache/twig'
end
end
end
```

My sudoers file looks like this:

```
Cmnd_Alias CLEAR_CACHE = /usr/bin/whoami, /bin/rm -rf src/cache/twig
Defaults!CLEAR_CACHE !requiretty
david ALL=(web1) NOPASSWD: CLEAR_CACHE
```

I've been trying to use the debug log level in both capistrano and sshkit to figure out what the difference is between these two, more specifically how the `as` syntax executes the command, but all I ever got was:

```
** Invoke clear_cache (first_time)
** Execute clear_cache
00:00 clear_cache
01 rm -rf src/cache/twig
01 sudo: no tty present and no askpass program specified
cap aborted!
SSHKit::Runner::ExecuteError: Exception while executing as david@remote-host: rm exit status: 1
rm stdout: Nothing written
rm stderr: sudo: no tty present and no askpass program specified
/home/kwisatz/1024/projects/a_project/.vendor/bundle/ruby/2.4.0/gems/sshkit-1.13.1/lib/sshkit/runners/parallel.rb:15:in `rescue in block (2 levels) in execute'
/home/kwisatz/1024/projects/a_project/.vendor/bundle/ruby/2.4.0/gems/sshkit-1.13.1/lib/sshkit/runners/parallel.rb:11:in `block (2 levels) in execute'
SSHKit::Command::Failed: rm exit status: 1
rm stdout: Nothing written
rm stderr: sudo: no tty present and no askpass program specified
/home/kwisatz/1024/projects/a_project/.vendor/bundle/ruby/2.4.0/gems/sshkit-1.13.1/lib/sshkit/command.rb:100:in `exit_status='
/home/kwisatz/1024/projects/a_project/.vendor/bundle/ruby/2.4.0/gems/sshkit-1.13.1/lib/sshkit/backends/netssh.rb:165:in `execute_command'
/home/kwisatz/1024/projects/a_project/.vendor/bundle/ruby/2.4.0/gems/sshkit-1.13.1/lib/sshkit/backends/abstract.rb:141:in `block in create_command_and_execute'
/home/kwisatz/1024/projects/a_project/.vendor/bundle/ruby/2.4.0/gems/sshkit-1.13.1/lib/sshkit/backends/abstract.rb:141:in `tap'
/home/kwisatz/1024/projects/a_project/.vendor/bundle/ruby/2.4.0/gems/sshkit-1.13.1/lib/sshkit/backends/abstract.rb:141:in `create_command_and_execute'
/home/kwisatz/1024/projects/a_project/.vendor/bundle/ruby/2.4.0/gems/sshkit-1.13.1/lib/sshkit/backends/abstract.rb:74:in `execute'
/home/kwisatz/1024/projects/a_project/Capfile:35:in `block (4 levels) in '
/home/kwisatz/1024/projects/a_project/.vendor/bundle/ruby/2.4.0/gems/sshkit-1.13.1/lib/sshkit/backends/abstract.rb:85:in `within'
/home/kwisatz/1024/projects/a_project/Capfile:34:in `block (3 levels) in '
/home/kwisatz/1024/projects/a_project/.vendor/bundle/ruby/2.4.0/gems/sshkit-1.13.1/lib/sshkit/backends/abstract.rb:112:in `as'
/home/kwisatz/1024/projects/a_project/Capfile:33:in `block (2 levels) in '
/home/kwisatz/1024/projects/a_project/.vendor/bundle/ruby/2.4.0/gems/sshkit-1.13.1/lib/sshkit/backends/abstract.rb:29:in `instance_exec'
/home/kwisatz/1024/projects/a_project/.vendor/bundle/ruby/2.4.0/gems/sshkit-1.13.1/lib/sshkit/backends/abstract.rb:29:in `run'
/home/kwisatz/1024/projects/a_project/.vendor/bundle/ruby/2.4.0/gems/sshkit-1.13.1/lib/sshkit/runners/parallel.rb:12:in `block (2 levels) in execute'
Tasks: TOP => clear_cache
```

How can I see what __exact__ command sshkit is running when using `as` ? I'd presume that it doesn't correspond to what I allowed in the sudoers file.