Generate a condition when value is false for boolean predicates

I noticed that for boolean predicates the value `false` is being ignored. This can be counterintuitive as I would expect that `name_present = false` would give me all records where `name` equals `NULL` or is empty.

I found this issue https://github.com/activerecord-hackery/ransack/issues/9 and implemented something starting from @cawel spec.

I am not sure if it is the best approach, but let me know what you think.