BUG: first() loses the timezone in groupby
need a systematic test of all groupby methods here.

This works as expected:

```
pd.DataFrame({'time': [pd.Timestamp('2012-01-01 13:00:00+00:00')],
              'A': [3]}).groupby('A', as_index=False).head(1)
```

However, this loses the timezone:

```
pd.DataFrame({'time': [pd.Timestamp('2012-01-01 13:00:00+00:00')],
              'A': [3]}).groupby('A', as_index=False).first()
```
