Integer Group Array Keys can create empty node forms

Thanks for submitting and working on this really useful module.

This is a pretty obscure error but it has occurred in conjunction with the scheduling and inline entity form modules.

The Group array contained an integer array key for the scheduling options which means that the in_array function returned false positives and consequently hid all the form fields from an inline node form.

Here's a patch to add a check for string array keys, and use strict in_array checking. Can this please be applied?

Thanks,

Dubs