Fix su bug where connecting as root would not allow su to another user

Hi guys,

This resolves the su issue as reported on the ansible-project mailing list.  The following playbook verifies the bug and the resolution with the patch.

```
- name: Test su connecting as root
  hosts: test
  gather_facts: no
  remote_user: root
  tasks:
    - name: Test connecting as root
      command: /usr/bin/whoami

    - name: Connect as root and su to testuser
      command: /usr/bin/whoami
      su: yes
      su_user: testuser
```
Tests are passing.
```
----------------------------------------------------------------------
Ran 96 tests in 27.138s

OK (SKIP=1)
```