Fix BC break in 2.1.5dev - Revert to previous isRequired behavior for file upload inputs

#3983 causes BC breaks for FilePostRedirectGet Plugin, and in existing projects using File Inputs with changed isRequired settings.

To reproduce, install https://github.com/cgmartin/ZF2FileUploadExamples and test the following examples:
* "Single File Upload with partial validation"
* "Post-Redirect-Get Plugin Example"
* "FilePRG Plugin Example with nested Fieldset"

With ZF2.1.4, the examples work correctly.
With ZF2.1.5dev, the examples fail.

Reported via email by @leup (Thanks!)