"Cannot convert tz-naive timestamps"
I'm not sure if it is an issue of DOCs or code.

[Docs](http://pandas.pydata.org/pandas-docs/dev/generated/pandas.DataFrame.tz_convert.html) say that `tz_convert` "Convert the axis to target time zone. _If it is time zone naive, it will be localized to the passed time zone_."

But the code in contrast raises an [exception](https://github.com/pydata/pandas/blob/bb9d35c27abdd68f349950c404292426f730759d/pandas/tseries/index.py#L1597):

```
tmp = pd.Series([0], index=[pd.Timestamp('2008-12-16 13:10:00')])
tmp.tz_convert('UTC')
```

Results in `TypeError: Cannot convert tz-naive timestamps, use tz_localize to localize`
