BUG: to_datetime not properly converting tz-aware datetime object
#### Code

input:
```python
from datetime import datetime
import pandas as pd
import pytz

pacific = pytz.timezone("US/Pacific")
notz = datetime(2018,1,1,0,0,0,0)
withtz = datetime(2018,1,1,0,0,0,0, pacific)

notz.strftime('%Y-%m-%d %H:%M:%S')
withtz.strftime('%Y-%m-%d %H:%M:%S')

pd.to_datetime(notz)
pd.to_datetime(withtz)
```

Output:
```
'2018-01-01 00:00:00'
'2018-01-01 00:00:00'
Timestamp('2018-01-01 00:00:00')
Timestamp('2017-12-31 23:53:00-0800', tz='US/Pacific')
```

Expected Output:
```
'2018-01-01 00:00:00'
'2018-01-01 00:00:00'
Timestamp('2018-01-01 00:00:00')
Timestamp('2017-01-01 00:00:00-0800', tz='US/Pacific')
```
#### Problem description

When passing a tz-aware datetime object to `to_datetime`, the pandas timestamp object alters the minutes of the object. 
