Fix UpdatePort corrupting the host when the port digits appear inside the host/IP

[SergoRoberto]

Closes #753

Fix UpdatePort corrupting the host when the port digits appear inside the host/IP

Problem

URL.UpdatePort replaced the port via strings.Replace(u.Host, u.Port(), newport, 1),
which swaps the first occurrence of the port substring in the host — not the actual
port. When the port digits also appear inside the IP/hostname, the wrong part gets rewritten.

Examples:

• http://37.228.93.80:80/ + UpdatePort("443") → http://37.228.93.443:80/ ❌
• http://[2001:db8::80]:80/ + UpdatePort("8080") → http://[2001:db8::8080]:80/ ❌

Fix

Build the host with net.JoinHostPort(u.Hostname(), newport). Hostname() strips the
existing port and unwraps IPv6 brackets, and JoinHostPort re-wraps IPv6 literals
correctly — so the port is always set on the right segment.

Tests

Added TestPortUpdateWhenPortMatchesHost covering IPv4 and IPv6 cases that previously
produced a corrupted host.