whoops, don't dereference all references as the first type encountered

tests/dereference.py

@@ -815,17 +815,29 @@ class FieldTest(unittest.TestCase):
         class Foo(Document):
             meta = {'allow_inheritance': False}
             bar = ReferenceField('Bar')
+            baz = ReferenceField('Baz')
 
         class Bar(Document):
             meta = {'allow_inheritance': False}
             msg = StringField(required=True, default='Blammo!')
 
+        class Baz(Document):
+            meta = {'allow_inheritance': False}
+            msg = StringField(required=True, default='Kaboom!')
+
         Foo.drop_collection()
         Bar.drop_collection()
+        Baz.drop_collection()
 
         bar = Bar()
         bar.save()
+        baz = Baz()
+        baz.save()
         foo = Foo()
         foo.bar = bar
+        foo.baz = baz
         foo.save()
         foo.reload()
+
+        self.assertEquals(type(foo.bar), Bar)
+        self.assertEquals(type(foo.baz), Baz)