Adapt to relative path bug fix in purell

Adapt to relative path bug fix in purell

Temporary workaround for the bug fix and resulting
behavioral change in purell.NormalizeURLString():
a leading '/' was inadvertently to relative links,
but no longer, see #878.

I think the real solution is to allow Hugo to
make relative URL with relative path,
e.g. "../../post/hello-again/", as wished by users
in issues #157, #622, etc., without forcing
relative URLs to begin with '/'.
Once the fixes are in, let's remove this kludge
and restore SanitizeUrl() to the way it was.

Fixes #878