API/BUG: center=True in expanding operations returns non-sensical results
What is `center=True` supposed to do in `df.expanding()...` operations?

Using the example from the docstring of `.expanding()`:

```
In [103]: df = DataFrame({'B': [0, 1, 2, np.nan, 4]})

In [105]: df.expanding(2).sum()
Out[105]: 
     B
0  NaN
1  1.0
2  3.0
3  3.0
4  7.0
```

Now adding `center=True`:

```
In [106]: df.expanding(2, center=True).sum()
Out[106]: 
     B
0  3.0
1  3.0
2  7.0
3  7.0
4  6.0
```

Two observations: it seems it just shifted the first rows (which is not the same as centering) and I don't know where the last values are coming from (and they should certainly not decrease again in an expanding sum with only positive values). 

Further, I also cannot think of what it actually *should* do. If we really want to center it, that would mean returning with a new index (as the real center would only increase with 0.5 and be something like [0, 0.5, 1, 1.5, 2] for those data).

So maybe we should rather remove this keyword for `expanding` ?