Revert to previous isRequired behavior for file uploads (fix BC break)
Remove unnecessary test for isRequired check
Merge branch 'hotfix/4168'

Close #4168
Fixes #4167
Merge pull request #4171 from cgmartin/fix/file-prg-base-input-filter

Fix BC break in 2.1.5dev - Revert to previous isRequired behavior for file upload inputs
Merge branch 'hotfix/4171' into develop

Forward port #4171

Conflicts:
	library/Zend/InputFilter/BaseInputFilter.php