[BUG][FIX] upToEnd in PositionableStream ( [er][sm] works as advertised )

[ken at kencausey.com]

This is an appropriate fix.  I can't think of a single logical user that
would intend for this method to only return the elements from the
current position up to the first nil element.  If a subclass wants a
different definition of what atEnd means than this changeset allows that
whereas the existing code has a fixed definiton of that it meeans to be
the end for this method.













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