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/json_serialisation.py

@@ -51,6 +51,10 @@ class TestJson(unittest.TestCase):
             string = StringField()
             embedded_field = EmbeddedDocumentField(Embedded)
 
+            def __eq__(self, other):
+                return (self.string == other.string and
+                        self.embedded_field == other.embedded_field)
+
         doc = Doc(string="Hi", embedded_field=Embedded(string="Hi"))
 
         doc_json = doc.to_json(sort_keys=True, separators=(',', ':'))
@@ -99,6 +103,10 @@ class TestJson(unittest.TestCase):
             generic_embedded_document_field = GenericEmbeddedDocumentField(
                                         default=lambda: EmbeddedDoc())
 
+            def __eq__(self, other):
+                import json
+                return json.loads(self.to_json()) == json.loads(other.to_json())
+
         doc = Doc()
         self.assertEqual(doc, Doc.from_json(doc.to_json()))