additional fixes to support Mongo3.4

tests/document/indexes.py

@@ -603,18 +603,24 @@ class IndexesTest(unittest.TestCase):
 
     @requires_mongodb_gte_34
     def test_primary_key_unique_not_working_under_mongo_34(self):
+        """Relates to  #1445"""
         class Blog(Document):
             id = StringField(primary_key=True, unique=True)
 
+        Blog.drop_collection()
+
         with self.assertRaises(OperationFailure) as ctx_err:
             Blog(id='garbage').save()
         self.assertIn("The field 'unique' is not valid for an _id index specification", str(ctx_err.exception))
 
     @requires_mongodb_lte_32
     def test_primary_key_unique_working_under_mongo_32(self):
+        """Relates to  #1445"""
         class Blog(Document):
             id = StringField(primary_key=True, unique=True)
 
+        Blog.drop_collection()
+
         Blog(id='garbage').save()
 
     def test_unique_with(self):