BUG: fillna() for tz-aware timestamps causes an exception
Suppose I have a DataFrame of timestamps:

```python
df = pd.DataFrame({'A': [pd.Timestamp('2012-11-11 00:00:00'),
                         pd.Timestamp('2012-11-11 00:00:00')]})
```

I can attempt to fill any nulls:

```python
In [23]: df.fillna(method='pad')
Out[23]:
           A
0 2012-11-11
1 2012-11-11
```

Now suppose my timestsamps are tz-aware:

```python
df = pd.DataFrame({'A': [pd.Timestamp('2012-11-11 00:00:00+00:00'),
                         pd.Timestamp('2012-11-11 00:00:00+00:00')]})
```

This causes an error:

```python
In [25]: df.fillna(method='pad')
...
ValueError: total size of new array must be unchanged
```

This does not appear to be fixed by #14960.
