cqlengine: fix map remove feature for non-nullable values

following code:
```
.update(
            bin_map__remove={123, 456}
        )
```
Would fail if map is defined with non-nullable value type:
```
columns.Map(columns.BigInt, columns.Bytes, required=False, default={})
```

Author: dkropachev

cassandra/cqlengine/statements.py

@@ -821,7 +821,9 @@ def update_context_id(self, i):
             self.context_counter += conditional.get_context_size()
 
     def add_update(self, column, value, operation=None, previous=None):
-        value = column.to_database(value)
+        # For remove all values are None, no need to convert them
+        if operation != 'remove':
+            value = column.to_database(value)
         col_type = type(column)
         container_update_type = ContainerUpdateClause.type_map.get(col_type)
         if container_update_type:

tests/integration/cqlengine/base.py

@@ -30,6 +30,8 @@ class TestQueryUpdateModel(Model):
     text_set = columns.Set(columns.Text, required=False)
     text_list = columns.List(columns.Text, required=False)
     text_map = columns.Map(columns.Text, columns.Text, required=False)
+    bin_map = columns.Map(columns.BigInt, columns.Bytes, required=False, default={})
+
 
 class BaseCassEngTestCase(unittest.TestCase):
 

tests/integration/cqlengine/query/test_updates.py

@@ -246,20 +246,30 @@ def test_map_update_remove(self):
         TestQueryUpdateModel.objects.create(
             partition=partition,
             cluster=cluster,
-            text_map={"foo": '1', "bar": '2'}
+            text_map={"foo": '1', "bar": '2'},
+            bin_map={123: b'1', 789: b'2'}
         )
         TestQueryUpdateModel.objects(partition=partition, cluster=cluster).update(
             text_map__remove={"bar"},
-            text_map__update={"foz": '4', "foo": '2'}
+            text_map__update={"foz": '4', "foo": '2'},
+            bin_map__remove={789},
+            bin_map__update={456: b'4', 123: b'2'}
         )
         obj = TestQueryUpdateModel.objects.get(partition=partition, cluster=cluster)
         self.assertEqual(obj.text_map, {"foo": '2', "foz": '4'})
+        self.assertEqual(obj.bin_map, {123: b'2', 456: b'4'})
 
         TestQueryUpdateModel.objects(partition=partition, cluster=cluster).update(
-            text_map__remove={"foo", "foz"}
+            text_map__remove={"foo", "foz"},
+            bin_map__remove={123, 456}
+        )
+        rec = TestQueryUpdateModel.objects.get(partition=partition, cluster=cluster)
+        self.assertEqual(
+            rec.text_map,
+            {}
         )
         self.assertEqual(
-            TestQueryUpdateModel.objects.get(partition=partition, cluster=cluster).text_map,
+            rec.bin_map,
             {}
         )