function to munge booleans to httpd's On/Off

this function tries to recognize booleans of any kind and convert
(normalize) them httpd's accepted On/Off.
Failing that, it will simply return the string it was passed. As such it
can be used, for instance for:
```puppet
    $server_tokens = true
    $server_tokens = false
    $server_tokens = 'mail'
```