Feature idea: boolean validator

I was thinking to implement a shorthand for validating boolean values.

Instead of doing (which we do right now):

validates :foo, inclusion: [true, false]

you could do the shorter and more idiomatic:

validates :foo, boolean: true

What do you think? If it proves a good idea I’ll start working on a PR.

Has been already discussed:

    https://groups.google.com/forum/#!msg/rubyonrails-core/HrxYkzUoJ34/ybdTecsvl9cJ

Makes sense. Thanks :slight_smile: