```python
m = Metadata(tag1='a', tag2='b')
del m['tag1']
```
```
m store: {'tag2': ['b']}
m deleted: {'tag1'}
```
```python
m2 = Metadata(tag1='c', tag2='d')
del m2['tag2']
```
```
m2 store: {'tag2': ['b']}
m2 deleted: {'tag1'}
```
```python
m.update(m2)
```
```
m store: {'tag1': ['c']}
m deleted: {'tag1', 'tag2'} <---- this was incorrect, and untested case
```
This patch fixes it, and results to:
```
m store: {'tag1': ['c']}
m deleted: {'tag2'}
```
2aec4a4185