Keep a // path from becoming an authority on unsplit

[gyanu2507]

scheme:/..///bar has no authority. Removing dot-segments leaves path //bar, and unsplit() then emits scheme://bar. Parsing that string treats bar as the host.

If there is no authority and the path starts with //, prefix /. so the reconstituted URI still has no host. Same idea as the WHATWG serializer for a path whose first segment is empty.

>>> from rfc3986 import uri_reference
>>> uri_reference("scheme:/..///bar").normalize().unsplit()
# before: "scheme://bar"
# after:  "scheme:/.//bar"
>>> uri_reference("scheme:/.//bar").authority
None