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_signature = true
    $server_signature = false
    $server_signature = 'mail'
```
@ekohl you mean this should check for `default`'s class to be string, rather than converting it