[Form] Allowed binding false to a checkbox

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | [![Build Status](https://travis-ci.org/jakzal/symfony.png?branch=checkbox-type-false)](https://travis-ci.org/jakzal/symfony)
| Fixed tickets | #7139
| License       | MIT
| Doc PR        | -