Accept strings as disabled value.

Old FormHelper was pretty permissive about disabled values. So
we can keep that going forward.
Add tests for Radio options and string disabled.
Re-enable tests for multi-checkbox.
Unskip a secured form test.

Fix issues with step and hidden fields as there were some problems.
Fix additional tests.

Schema data has length/precision separated now.
Merge pull request #2912 from cakephp/3.0-form-checkbox

3.0 form checkbox