(FACT-351) Fix netmask fact on Darwin and *BSD

During facter2 development, the method_missing logic for resolving fact
values has been removed, e.g. Facter.ipaddress. There was one case left
over that when called would produce:

    Could not retrieve fact='netmask', resolution='<anonymous>':
    undefined method `ipaddress' for Facter:Module

and no backtrace.

This commit patches up the fact and adds a spec test.