BUG: Series.str.cat(series) loses original name
0.22.0

```python
In [6]: pd.Series(['a', 'b'], name='foo').str.cat(pd.Series(['a', 'b']), sep=':').name
Out[6]: 'foo'
```

master

```python
In [2]: pd.Series(['a', 'b'], name='foo').str.cat(pd.Series(['a', 'b']), sep=':').name
```

fixing now.