datetimeindex.to_period not behaving as expected. 
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
from datetime import datetime as dt

dates = [
            dt(2000, 1, 1, 0, 0),
            dt(2000, 1, 1, 0, 0, 30),
            dt(2000, 1, 1, 0, 1),
            dt(2000, 1, 1, 0, 1, 30),
            dt(2000, 1, 1, 0, 2),
            dt(2000, 1, 1, 0, 2, 30),
            dt(2000, 1, 1, 0, 3),
            dt(2000, 1, 1, 0, 3, 30),
            dt(2000, 1, 1, 0, 4),
            dt(2000, 1, 1, 0, 4, 30),
            dt(2000, 1, 1, 0, 5),
            dt(2000, 1, 1, 0, 5, 30),
            dt(2000, 1, 1, 0, 6),
            dt(2000, 1, 1, 0, 6, 30)]

idx = pd.DatetimeIndex(dates)
idx.to_period('5T')
```
#### Problem description

By changing the period to 5 minutes, I expected the output to be:

```
PeriodIndex(['2000-01-01 00:00', '2000-01-01 00:00', '2000-01-01 00:00',
             '2000-01-01 00:00', '2000-01-01 00:00', '2000-01-01 00:00',
             '2000-01-01 00:00', '2000-01-01 00:00', '2000-01-01 00:00',
             '2000-01-01 00:00', '2000-01-01 00:05', '2000-01-01 00:05',
             '2000-01-01 00:05', '2000-01-01 00:05'],
            dtype='period[5T]', freq='5T')
```

but it is 

```
PeriodIndex(['2000-01-01 00:00', '2000-01-01 00:00', '2000-01-01 00:01',
             '2000-01-01 00:01', '2000-01-01 00:02', '2000-01-01 00:02',
             '2000-01-01 00:03', '2000-01-01 00:03', '2000-01-01 00:04',
             '2000-01-01 00:04', '2000-01-01 00:05', '2000-01-01 00:05',
             '2000-01-01 00:06', '2000-01-01 00:06'],
            dtype='period[5T]', freq='5T')
```

Which makes it look like the actual period is one minute ('T') since thats what the output corresponds to, but the freq shows correctly as 5T. 

Have I misunderstood what `to_period` should return? or is this a bug?

I am using the latest version of pandas (0.23.4)