Explicitly name ids of clashing base models

This avoids the following error in django 1.11 tests:

    polymorphic.MRODerived: (models.E005) The field 'id' from parent model 'polymorphic.mrobase3' clashes with the field 'id' from parent model 'polymorphic.mrobase1'.

Related to https://code.djangoproject.com/ticket/22442

polymorphic/tests/__init__.py

@@ -196,6 +196,7 @@ if django.VERSION >= (1, 7):
 
 class MROBase1(ShowFieldType, PolymorphicModel):
     objects = MyManager()
+    base_1_id = models.AutoField(primary_key=True)
     field1 = models.CharField(max_length=10)  # needed as MyManager uses it
 
 
@@ -204,6 +205,7 @@ class MROBase2(MROBase1):
 
 
 class MROBase3(models.Model):
+    base_3_id = models.AutoField(primary_key=True)
     objects = PolymorphicManager()