BUG: to_datetime(Timestamp) utc argument

```
In [7]: pd.to_datetime(pd.Timestamp(2018,1,1), utc=True)
Out[7]: Timestamp('2018-01-01 00:00:00')

# This should raise
In [7]: pd.to_datetime(pd.Timestamp(2018,1,1, tz='US/Pacific'), utc=True)
Out[7]: Timestamp('2018-01-01 00:00:00')
```