Allow to specify a port from the conn

[kpanic]

Hey there 👋🏾

Currently the port can be specified by an URI like http://localhost:4200 but in phoenix it's required to do a:

get(conn, "http://localhost:42000/my/path", ...)

Because we do an URI.parse/1 here https://github.com/elixir-plug/plug/blob/main/lib/plug/adapters/test/conn.ex#L9

Instead the port in the conn struct could be change and that should be cleaner rather than specifing a full URI, in phoenix tests.

Thanks for creating Plug!!

PS: Hope that this PR makes sense 😉

[josevalim]

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