PeriodIndex with float input inconsistency
#### Code Sample, a copy-pastable example if possible

```
pd.PeriodIndex([1.1], freq='M')
# ValueError: Given date string not likely a datetime.

pd.PeriodIndex(np.array([1.1]), freq='M')
# PeriodIndex(['1970-02'], dtype='int64', freq='M'
```
#### Expected Output

Consistently raise or coerce to `Period`
#### output of `pd.show_versions()`

0.18.1

CC: @MaximilianR
