Difference in groupby behavior between Pandas 0.13.1 and 0.15.2
Hi, I am seeing a difference in behavior on this groupby between  Pandas 0.13.1 and 0.15.2.  Specifically, it's like 0.15.2 is doing a cross join while 0.13.1 isn't.

```
print pandas.DataFrame([
  {'a': 1, 'b': 2, 'c': 3},
  {'a': 4, 'b': 5, 'c': 6}, ]).set_index(
    list('ab')).groupby(level=list('ab')).mean()
```

0.13.1 produces:

```
     c
a b   
1 2  3
4 5  6
[2 rows x 1 columns]
```

while 0.15.2 produces

```
      c
a b    
1 2   3
  5 NaN
4 2 NaN
  5   6
```

basically, the same matrix, but with extra cross NaN entries.

We're wondering if this behavior is intentional, or a bug.  It wasn't entirely clear from the set of release notes that the groupby behavior changed so much.
