Correct implementation of uri.origin and add tests for default ports

[nijolas]

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uri.origin should not include a trailing slash - I've corrected the relevant tests and updated the function to concat its own results, as this behaviour appears specific to the origin function only, not to_string.

Additionally, I've added two tests to cover the default port stripping when http or https are supplied along wtih port 80 or 443 respectively.

Fixes https://github.com/gleam-lang/stdlib/issues/623.