Add tests for proper Checkbox checked/unchecked value handling
Allow Checkbox to return the intended value instead of a boolean when checked/unchecked
Merge branch 'hotfix/4528'

Close #4528
Merge pull request #4529 from sjuvonen/master

Allow Zend\Form\Element\Checkbox to return real value instead of always a boolean
Merge branch 'hotfix/4529' into develop

Forward port #4529