BUG: use astype for scipy datetime interpolation

Closes #5975 (again), with the issue raised at https://github.com/pydata/pandas/pull/5977#issuecomment-32691271.

I'm using `astype` instead of `view` now, as Jeff suggested. I also didn't realize that `asi8` was a method on all `Index`es, and not just `DatetimeIndex`es. So instead of checking for that attr, I just check if it's a DatetimeIndex.

@y-p this should fix the doc example. It built for me anyway, but I've had trouble building the docs off my working branch.