(maint) Make setting not screw up when interpolating a false value

This was a class nil/false screwup.
(PUP-542) Add structured facts to immutable $facts

When opting in with trusted_node_data or immutable_node_data the facts
are also available via $facts, an immutable hash of data.
(PUP-542) Improve descriptions of trusted & immutable node data settings
(maint) Add test for interpolation of setting having false value
(PUP-542) Make tests pass on 1.8.7 ('hash' vs 'Hash' in message)
Merge pull request #2389 from hlindberg/pup-542_structured-facts-support

(PUP-542) Add structured facts support