Better BaseDocument equality check when not saved

When 2 instances of a Document had id = None they would be considered
equal unless an __eq__ were implemented.

We now return False for such case. It now behaves more similar to
Django's ORM.

tests/document/instance.py

@@ -2503,6 +2503,10 @@ class InstanceTest(unittest.TestCase):
             doc_name = StringField()
             doc = EmbeddedDocumentField(Embedded)
 
+            def __eq__(self, other):
+                return (self.doc_name == other.doc_name and
+                        self.doc == other.doc)
+
         classic_doc = Doc(doc_name="my doc", doc=Embedded(name="embedded doc"))
         dict_doc = Doc(**{"doc_name": "my doc",
                           "doc": {"name": "embedded doc"}})
@@ -2519,6 +2523,10 @@ class InstanceTest(unittest.TestCase):
             doc_name = StringField()
             docs = ListField(EmbeddedDocumentField(Embedded))
 
+            def __eq__(self, other):
+                return (self.doc_name == other.doc_name and
+                        self.docs == other.docs)
+
         classic_doc = Doc(doc_name="my doc", docs=[
                           Embedded(name="embedded doc1"),
                           Embedded(name="embedded doc2")])
@@ -2719,5 +2727,16 @@ class InstanceTest(unittest.TestCase):
         self.assertEquals(p4.height, 189)
         self.assertEquals(Person.objects(height=189).count(), 1)
 
+    def test_not_saved_eq(self):
+        """Ensure we can compare documents not saved.
+        """
+        class Person(Document):
+            pass
+
+        p = Person()
+        p1 = Person()
+        self.assertNotEqual(p, p1)
+        self.assertEqual(p, p)
+
 if __name__ == '__main__':
     unittest.main()