concat of tz series and no tz results in attribute error.
``` python
import pandas as pd
x = pd.Series(pd.date_range('20151124 10:00', '20151124 11:00', freq = '1h', tz = "UTC") )
y = pd.Series(pd.date_range('2012-01-01', '2012-01-03'))
pd.concat([x,y])
```

AttributeError: 'numpy.ndarray' object has no attribute 'tz_localize'

Is it a bug or not supported ?
