fix: port property leaks stale scheme default when scheme changes

[gaoflow]

The port setter stores DEFAULT_PORTS.get(self.scheme) in self._port when port=None. This caches the scheme-dependent default port, which becomes stale when the scheme later changes -- the port property returns the cached value instead of the new scheme's default.

Concrete reproduction:

from furl import furl

f = furl('https://www.google.com/hello')
f.copy().set(scheme=None, netloc=None).url
# Expected: '/hello'
# Actual:   '//:443/hello'  (port 443 leaks from the old https scheme)

Root cause: netloc=None processes before scheme=None in set(), so port=None caches DEFAULT_PORTS['https']=443 before the scheme is cleared.

Fix: store None in _port when port=None, letting the port getter's existing self._port or DEFAULT_PORTS.get(self.scheme) logic supply the scheme-appropriate default dynamically at access time. This also fixes the case where port=None followed by scheme='https' previously returned port 80 instead of 443.

Also removes the now-unnecessary _port cache assignment in load().

Fixes #143.