Don't require default-directory to end with a slash

Don't require default-directory to end with a slash

* doc/lispref/files.texi (Magic File Names): Document the change
in unhandled-file-name-directory.

* lisp/url/url-handlers.el
(url-handler-unhandled-file-name-directory): Update accordingly.

* src/buffer.c (default-directory): Update the docsting.

* src/fileio.c (unhandled-file-name-directory): Default to calling
`file-name-as-directory'
(http://lists.gnu.org/archive/html/emacs-devel/2015-10/msg02294.html).