fix for unselected select multiple; added options use_hidden_element and unselected_value
fix for unselected select multiple; unit test for form
fix for unselected select multiple; dummy select if select multiple is unselected
fix for unselected select multiple; fix docblock for consistency
fix for unselected select multiple; extended the unit test, fix doc block, fix margin
Merge branch 'hotfix/5997'

Close #5997
Fixes #5965
Merge pull request #6009 from tommyseus/master

Form\Element\Select multiple is always required
Merge branch 'hotfix/6009' into develop

Forward port #6009