Concat Series[datetimetz] & Series[category] loses timezone
```python

In [8]: a = pd.Series(pd.date_range('20170101', periods=4, tz='US/Pacific'))

In [9]: b = pd.Series(['a', 'b'], dtype='category')

In [10]: pd.concat([a, b], ignore_index=True)[0]
Out[10]: Timestamp('2017-01-01 08:00:00')

```

Out[10]  should have a tz like the following

```python
In [11]: pd.concat([a, b.astype(object)], ignore_index=True)[0]
Out[11]: Timestamp('2017-01-01 00:00:00-0800', tz='US/Pacific', freq='D')
```