Fix for error when using tests.pp

Applying `tests.pp` without this commit will leave `Printer_A` without model or PPD file. On the second run, Puppet will complain

    Error: Could not prefetch printer provider 'cups': undefined method `captures' for nil:NilClass

because the call to `lpoptions -p Printer_A -l` returned

    lpoptions: Unable to get PPD file for Printer_A: Not Found

Since the module does not (yet) verify or sanitize user input, I'll just suggest a fix here.

Note also, that the `tests.pp` passes (at least in Ubuntu 14.04 with CUPS 1.7.2) despite the apostrophe in the `location` property. Prefetch as well as `puppet resource printer` work on this example without error. Therefore I suggest to close issue #7.