Redirect to start page for the root namespace too

Dokuwiki emits an HTTP redirect when accessing:
/path/to/some/namespace/

to force the browser to:

/path/to/some/namespace/$conf['start']

However, this doesn't happen for the root namespace itself.

In some ways, this doesn't really matter, since accessing plain
"doku.php" will show the root namespace "start" page even without the
redirection. Equally, this can be trivially fixed using mod_rewrite
rules, but I don't want to encode the start page name into my rewrite
rules. Finally, I like the consistency of always having the start page
name in the URL, rather than special-casing the root namespace.
(Actually, I'd prefer never to have it in the URL, but that looks more
complex to achieve).

Anyway, the attached patch makes the redirect happen for the root
namespace as well.