to_datetime() has an unexpected output when time is 00:00:00
Hi, I got a problem that when I use to_datetime() function on a special timestamp, for example: 1531440000
it should be 2018-07-13 00:00:00, but I just got 2018-07-13. I think the result should be the first one even if it means the same datetime. So I assume all of the timestamp which can be convert to YYYY-mm-dd 00:00:00 is YYYY-mm-dd. Does I make something wrong or it's indeed a bug?
Code:
```
ary = np.array([1531440000])
result = pd.to_datetime(ary, unit='s')
print(result)
```

output:
2018-07-13

version:
0.18.1
