(PUP-665) Allow settings to be undefined

Setting a value to nil used to result in falling back to the default (or
whatever came later in the lookup chain). This behavior was lost when
the settings were pulled apart into multiple classes to handle the
lookup. The error showed up in tests for stdlib that use rspec-puppet,
which depended on the "nil is default" behavior.
Merge pull request #2174 from zaphod42/maint/master/allow-settings-to-be-undefined

(PUP-665) Allow settings to be undefined