Support other entities to be a group.

I've made user entity to be a group. And after node creation and assigning to user group I've got a couple of notices/warnings:

• Notice: Undefined index: node in og_workflow_user_roles_for_node() (line 585 of /var/www/invotra/sites/all/modules/contrib/og_workflow/og_workflow.module).
• Warning: array_values() expects parameter 1 to be array, null given in og_workflow_user_roles_for_node() (line 585 of /var/www/invotra/sites/all/modules/contrib/og_workflow/og_workflow.module).
• Warning: Invalid argument supplied for foreach() in og_workflow_user_roles_for_node() (line 585 of /var/www/invotra/sites/all/modules/contrib/og_workflow/og_workflow.module).

These errors appeared due to og_workflow module support only nodes to be a group. I've created simple patch to fix this.