Changed the default value of checkbox and radio to match the HTML spec

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | n/a

When the checkbox or radio input does not have a value attribute, the [HTML spec](http://www.whatwg.org/specs/web-apps/current-work/multipage/states-of-the-type-attribute.html#checkbox-state-%28type=checkbox%29) defines that the value [should be ``on``](http://www.whatwg.org/specs/web-apps/current-work/multipage/common-input-element-attributes.html#dom-input-value-default-on), not ``1``.

This change is a small BC break as a testsuite with an assertion on the ``1`` value would fail after the change. However, this means that such testsuite would assert a different behavior than the real usage of the application.
Note that people using the DomCrawler component on pages using the Symfony Form component are not affected by the change as the Form component always set an explicit value on checkboxes and radio buttons