Named routing _url doesn't work in dev/test (missing port)

When a named route is generated, the port is missing.

```elixir
# config/dev.exs
use Mix.Config

config :phoenix,
  routers: [
    [
      endpoint: MyApp.Router,
      port: 4000,
      host: "localhost",
      ssl: false,
    ]
  ]
```

```elixir
MyApp.Router.users_url == "http://localhost/users"
```

This can be fixed changing the code `/lib/phoenix/router/mapper.ex` but then it doesn't work when we use a proxy (likely to happen in production).

Maybe we can add a  `proxy` field in the configuration.
Any advice?