ActionController::Parameters#require should accept false boolean values

as reported in https://github.com/rails/rails/issues/15685#issuecomment-46009045
I suspect we should have a direct test on AC::Parameters for `false`, to match up with, and directly contrast against, the `nil` one. (Also, to confirm we're returning `false` -- it doesn't seem difficult for a poor implementation of this functionality to accidentally return `nil` in this case.)