years-only in date_range uses current date
I ran this on master current as of 2014-04-20 or so and was surprised as to the result:

``` python
In [17]: pd.date_range('2014', '2015', freq='M')
Out[17]: 
<class 'pandas.tseries.index.DatetimeIndex'>
[2014-04-30, ..., 2015-03-31]
Length: 12, Freq: M, Timezone: None
```

I would have expected that to give me the month-ends in 2014, not a year's worth of month-ends starting from today.

Seems to be related to #3944?
