Currently, #pluck in memory (i.e. on embedded docs) compacts the result before returning, i.e. it removes any `nil` values.

This is undesirable because:

1. nil values are useful information, for example, if I want to compare # of nil vs. non-nil.
2. The number of plucked results returned may not match the number of documents.
3. Mongoid::Criteria#pluck (aggregable) does return nil values, so memory pluck is not consistent with Mongoid.
4. ActiveSupport's Array#pluck does return nil values.