Allow null, present and blank accept false values

[artm]

otherwise it is difficult to have tristate filters (e.g. present,
absent, both) driven from a single simple control (e.g. select)

Fixes #353

[artm]

I had to extend the predicate options convention: now arel_predicate can be a proc accepting a typecast but yet unformatted value. The latter two commits implent the actual functionality with tests. None of the tested behavior got broken.

[jonatack]

Thank you @artm.