Added a forgotten super call for admin.NestedModelAdmin.get_inline_instances and misuse

[ppython]

Maybe a noob problem, but I got an empty list when doing:

class ExampleAdmin(NestedModelAdmin, SpecificAdmin1, SpecificAdmin2):
    def get_inline_instances(self, request, obj=None):
        inlines_inst = super(
            ExampleAdmin, self).get_inline_instances(request, obj)

Instead of getting an Inline from SpecificAdmin2...

With this patch, the MRO resolution of get_inline_instances is different and I finally get my Inline from SpecificAdmin2!

Is is something desirable for all cases?