Update FormHelper.php

disabled multiple checkbox input should not generate an enabled hidden input,
as it leads to saving empty data upon submission -> carrying the disabled attribute to the hidden input prevents this
fixing testcases + do not disable the hidden input on partial disables
Merge pull request #4975 from ptica/disabled-hidden-input

Disable the hidden input for multi checkbox inputs when inputs are disabled.
Merge branch 'master' into 2.6