Series/DataFrame.rename(<mapper>, level=None) doesn't work in non-ambiguous case
#### Code Sample, a copy-pastable example if possible

```python
# Your code here
>>> import pandas as pd
>>> pd.__version__
'0.26.0.dev0+1717.g6437f5eb3'
>>> s = (
...     pd.DataFrame({0: [1, 2, 3], 1: [4, 5, 6]})
...     .unstack()
...     .rename_axis([(1,), None])
... )
>>> s
(1,)
0     0    1
      1    2
      2    3
1     0    4
      1    5
      2    6
dtype: int64
>>>
>>> s.rename({0: "foo"}, level=None)
(1,)
foo   foo    1
      1      2
      2      3
1     foo    4
      1      5
      2      6
dtype: int64
```
#### Problem description

Is this an edge case or not supported.
 
#### Expected Output

```python
>>> s.rename({0: "foo"}, level=1)
(1,)
0     foo    1
      1      2
      2      3
1     foo    4
      1      5
      2      6
dtype: int64

```
#### Output of ``pd.show_versions()``

<details>

[paste the output of ``pd.show_versions()`` here below this line]

</details>
