Pandas multiindex unstack fillna not working
When we unstacking the multi-indexed pandas `dataframe`, the method `fillna` does not work.

Here is an example.

```python
import numpy as np
import pandas as pd

df = pd.DataFrame(np.random.randint(0,10,(5,4)),
                 columns=['c1','c2','c3','c4'])
df.iloc[1,2] = np.nan
df.iloc[0,0] = np.nan
df['ind1'] = ['a','a','b','b','c']
df['ind2'] = [1,2,1,2,1]
df = df.set_index(['ind1','ind2'])
print(df)
```
Now we have `df` with some `NaN` values.
```
            c1  c2   c3  c4
ind1 ind2                  
a    1     NaN   2  1.0   9
     2     9.0   5  NaN   7
b    1     1.0   7  2.0   8
     2     3.0   6  0.0   2
c    1     9.0   6  6.0   6
```
Then fillna to the unstacked `df` does not work.
```python
print(df.unstack().fillna(0))
```
```
       c1        c2        c3        c4     
ind2    1    2    1    2    1    2    1    2
ind1                                        
a     0.0  9.0  2.0  5.0  1.0  0.0  9.0  7.0
b     1.0  3.0  7.0  6.0  2.0  0.0  8.0  2.0
c     9.0  0.0  6.0  NaN  6.0  0.0  6.0  NaN
```
I raised this issue at [stack overflow](https://stackoverflow.com/questions/66692036/pandas-multiindex-unstack-fillna-not-working), and I suspect this is a bug.

I would appreciate to fix this problem.

Thanks,

> **Note**
> The `pandas` version is `1.1.3` (current version in Anaconda), and there is no issue with the version `==1.2.1`.