fix: return false instead of nil for reference types, when value is false-y

[mzikherman]

Noticing an odd bug where if a field has type: :reference and the definition is something that returns false, the value of nil gets returned instead of the correct false.

👋 @dblock

[dblock]

Hello there! I think we need to do #27 before we start merging other changes. Maybe add Mongoid 8 support? Care to do that?

[mzikherman]

Yup, I can do that. I was noticing the lack of CI yesterday, I ran the spec suite locally but just with the specific version of Ruby/Mongoid I happened to be using and was curious to see the matrix run w/ multiple.