fix-#620: saving document doesn't create new fields in existing collection

tests/document/instance.py

@@ -2633,5 +2633,15 @@ class InstanceTest(unittest.TestCase):
         p3 = Person.objects().only('created_on')[0]
         self.assertEquals(orig_created_on, p3.created_on)
 
+        class Person(Document):
+            created_on = DateTimeField(default=lambda: datetime.utcnow())
+            name = StringField()
+            height = IntField(default=189)
+
+        p4 = Person.objects()[0]
+        p4.save()
+        self.assertEquals(p4.height, 189)
+        self.assertEquals(Person.objects(height=189).count(), 1)
+
 if __name__ == '__main__':
     unittest.main()