bug #1748

This is a protect and unprotect test that exercises string series and block series.  It checks not only that errors are raised, but that the contents of the series are not modified while protected.

The first pass is done while the series are protected, and the second when protection has been removed.

If there's a more elegant way to handle the binding situation for the `series` variable, feel free to update it.  I never can figure out the magic incantation.  So I just use a placeholder and replace it in a copy at the moment.