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.

this might break for a PeriodIndex
you can do

getattr(x,'is_all_dates',False)

instead of doing isinstance which will catch both types of indexes 
 Thanks, I forgot about PeriodIndices. There's no need for a release notes entry right?
 nope
  Thanks, added a regression test.
