API/DOC: to_series keep_tz behaviour for UTC timezone
The docstring of `to_series` says for `keep_tz=True`: _"If the timezone is not set or is UTC, the resulting Series will have a datetime64[ns] dtype. Otherwise the Series will have an object dtype."_

http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DatetimeIndex.to_series.html

But is seems that also for UTC, `keep_tz=True` does the conversion to object/Timestamp?

```
In [1]: idx = pd.date_range('1/1/2000', periods=20, freq='1H', tz='UTC')

In [2]: idx
Out[2]:
<class 'pandas.tseries.index.DatetimeIndex'>
[2000-01-01 00:00:00+00:00, ..., 2000-01-01 19:00:00+00:00]
Length: 20, Freq: H, Timezone: UTC

In [3]: idx.to_series()[0]
Out[3]: Timestamp('2000-01-01 00:00:00')

In [4]: idx.to_series(keep_tz=True)[0]
Out[4]: Timestamp('2000-01-01 00:00:00+0000', tz='UTC', offset='H')
```
