Added field to test model to cause failure on PostgreSQL

Create a test that illustrates django#12728.

This is a bug that manifests when there's a generic relation to
a model that uses multi-table inheritence, with PostgreSQL as the
back-end.

tests/regressiontests/backends/models.py

@@ -1,3 +1,4 @@
+from django.contrib.comments.models import Comment
 from django.contrib.contenttypes import generic
 from django.contrib.contenttypes.models import ContentType
 from django.db import models, connection
@@ -44,10 +45,15 @@ class Tag(models.Model):
     content_object = generic.GenericForeignKey('content_type', 'object_id')
 
 
+class PossibleSelfComment(Comment):
+    by_author = models.BooleanField(default=False)
+
+
 class Post(models.Model):
     name = models.CharField(max_length=30)
     text = models.TextField()
     tags = generic.GenericRelation('Tag')
+    comments = generic.GenericRelation('PossibleSelfComment')
 
     class Meta:
         db_table = 'CaseSensitive_Post'