client: don't synthesize the default port into the Host header

[krynju]

Found this issue when porting to HTTP.jl 2 and AWS S3 presigned links were not working as is.

The problem

For a URL without an explicit port, the client puts the scheme's default
port into the Host header:

HTTP.get("https://example.com/")
# sends:  Host: example.com:443

The Host header should mirror the URL's authority as written. A bare-host
URL should send a bare Host: example.com. HTTP.jl 1.x did this; 2.0 regressed
because request.host is now built from the connection address
(example.com:443), which always carries a port for dialing.

Why it matters

Host: example.com:443 is technically legal (RFC 9110 §7.2 allows a port), but
many servers compare the Host value verbatim and a synthesized default port
breaks them. The case that bit us is AWS S3 presigned URLs (SigV4):

• The signature is computed over the canonical host s3.amazonaws.com.
• HTTP.jl 2.0 sends Host: s3.amazonaws.com:443.
• S3 recomputes the signature from the received Host, gets a different value,
  and rejects the request: 403 SignatureDoesNotMatch.

The same request succeeds with curl and with HTTP.jl 1.x, which send a bare
Host.

What other clients do

Go's net/http keeps the Host header equal to the URL authority and adds the
default port only to the dial address, never to Host:

req, _ := http.NewRequest("PUT", "https://s3.amazonaws.com/b/k", body)
httputil.DumpRequestOut(req, false)
//   PUT /b/k HTTP/1.1
//   Host: s3.amazonaws.com            <- bare; :443 not added

req2, _ := http.NewRequest("PUT", "https://s3.amazonaws.com:443/b/k", body)
//   Host: s3.amazonaws.com:443        <- explicit port preserved

curl and HTTP.jl 1.x behave the same way. HTTP.jl 2.0 is the outlier.

The fix

Add a host_header view on the parsed URL that mirrors the authority as
written — an explicit port is preserved, the default port is never synthesized
(and IPv6 brackets are kept) — and build request.host from it. The connection
address is unchanged, so dialing still targets the correct port.

URL	dial address (unchanged)	Host header (before → after)
https://example.com/	example.com:443	example.com:443 → example.com
https://example.com:443/	example.com:443	example.com:443 → example.com:443
http://minio:9000/	minio:9000	minio:9000 → minio:9000
https://[2001:db8::1]/	[2001:db8::1]:443	[2001:db8::1]:443 → [2001:db8::1]

Unit tests added in test/http_client_tests.jl covering bare host, explicit
default port, http default port, a custom port, and IPv6 (bracketed) forms.

Verified end-to-end: a real S3 presigned PUT that returns 403 on current
master returns 200 with this change.

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.72%. Comparing base (a1eb82b) to head (22eee75).

Files with missing lines	Patch %	Lines
src/http_client_url.jl	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1318      +/-   ##
==========================================
+ Coverage   87.67%   87.72%   +0.04%     
==========================================
  Files          30       30              
  Lines       11739    11746       +7     
==========================================
+ Hits        10292    10304      +12     
+ Misses       1447     1442       -5     

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