Fix polymorphic test

One assertion comment says look for association with dog but the code
still uses cat. This aligns the code to now look at the association of
the dog.

polymorphic_test.go

@@ -44,7 +44,7 @@ func TestPolymorphic(t *testing.T) {
 		t.Errorf("Cat's toys count should be 1")
 	}
 
-	if DB.Model(&cat).Association("Toy").Count() != 1 {
+	if DB.Model(&dog).Association("Toys").Count() != 2 {
 		t.Errorf("Dog's toys count should be 2")
 	}