DateRange union bug caused by bad DateOffset comparison
Reported by user

```
In [35]: rng1 = DateRange('12/5/2011', '12/5/2011')

In [36]: rng2 = DateRange('12/2/2011', '12/5/2011')

In [37]: rng1.union(rng2)
Out[37]: 
<class 'pandas.core.daterange.DateRange'>
offset: <1 BusinessDay>, tzinfo: None
[2011-12-02 00:00:00, ..., 2011-12-05 00:00:00]
length: 2

In [38]: rng2.offset = datetools.BDay()

In [39]: rng1.union(rng2)
Out[39]: Index([2011-12-02 00:00:00, 2011-12-05 00:00:00], dtype=object)
```
