Potential issue: default-port normalization depends on scheme case ("HTTP://example.org:80/" keeps its port)

[Sun-Wukhan]

Raising this here per the contributing guide, as PR creation on this repo is limited to collaborators. Found with property-based testing (round-trip and normalization invariants over generated URLs).

Default-port normalization currently depends on the case of the scheme, because urlparse() passes the raw scheme to normalize_port() instead of the lowercased one:

>>> import httpx
>>> httpx.URL("HTTP://example.org:80/") == httpx.URL("http://example.org/")
False
>>> httpx.URL("HTTP://example.org:80/")
URL("http://example.org:80/")  # port 80 kept, and re-parsing this string drops it

The string form of the first URL is not even a fixpoint: str(u) re-parses to a different URL, and copy_with() silently drops the port. The URL class docstring presents an uppercase scheme ("HTTPS://...") as parsing into normalized components, and states that the scheme "is normalized to always be lowercased" — the port lookup is the one place that still uses the un-normalized form.

Schemes are case-insensitive and are normalized to lowercase everywhere else in the parser, so the port lookup should use the normalized scheme too.

I have the one-line fix plus a regression test ready on a fork branch: https://github.com/Sun-Wukhan/httpx/tree/fix/url-scheme-case-port (the full tests/models suite passes with it). Happy for a maintainer to open the PR from it, or to open one myself if the restriction is lifted for it.