bug in deep copy of Series with timezone-aware datetime entries
deep copies of Series with timezone-aware datetimes are shallow in 0.17.1:

``` python
In [12]: import pandas as pd
In [13]: pd.__version__
Out[13]: u'0.17.1'

In [14]: ser = pd.Series([pd.Timestamp('2012/01/01', tz='UTC')])
In [15]: ser2 = ser.copy(deep=True)
In [18]: ser2[0] = pd.Timestamp('1999/01/01', tz='UTC')

# unexpected mutation
In [19]: ser
Out[19]: 
0   1999-01-01 00:00:00+00:00
dtype: datetime64[ns, UTC]
```
