Panel truncate failure
user reported this error (may be for all panels)

```
panel.truncate(before=None, after=None,
axis=’items’).fillna(value=0.0) will fail with an AssertionError

The failure path is pretty narrow.  If ‘before’ or ‘after’ is
binding, it won’t fail.  If the axis is ‘major’ or ‘minor’ it
won’t fail.  If fillna method is ‘pad’, it won’t fail.  So, it
seems to be just this pretty narrow set of parameter values that
causes a problem (i.e. reindexing the items of a Panel that
results in no change to the Index).
```
