Update issue #65 - remove_filename() post condition is incorrect

[EricWF]

Hi Beman,

The current issue resolution says:

Effects: *this = parent_path(), except that if parent_path() == root_path(), clear().

But this should say root_name() instead of root_path(). For example the breakdown of /foo is:
root_name = ""
root_path = "/"
parent_path = "/"

Using root_name() means that path("/foo").remove_filename() should call clear.