BUG/MINOR: http-htx: Support default path during scheme based normalization

BUG/MINOR: http-htx: Support default path during scheme based normalization

As stated in RFC3986, for an absolute-form URI, an empty path should be
normalized to a path of "/". This is part of scheme based normalization
rules. This kind of normalization is already performed for default ports. So
we might as well deal with the case of empty path.

The associated reg-tests was updated accordingly.

This patch should fix the issue #2573. It may be backported as far as 2.4 if
necessary.