Yet another thread on boolean validations

[Justin Kenyon]

I see there has been lots of chatter over the years on how to properly validate the presence of a boolean field.

The community has settled, rightly, on this

validates :field, inclusion: { in: [true, false] }
# or
validates :field, exclusion: { in: [nil] }

I'd like to suggest that we make this even better and add a `boolean` validator to allow for: 

validates :field, boolean: true

By using this method we would not only validate presence, but also the inclusion of a `true` or `false` value.

Would love to hear thoughts on this. PR incoming.