fix the wrong path configuration in root redirection

[MasashiSalvador57f]

What is included

• tiny bug fix for the configuration of root redirection

As before

when you setup root redirection for the gateway or endpoint, you can configure RootRedirect in your configuration file as

  "Gateway": {
   .... // omitted ...
    "NoFetch": false,
    "PathPrefixes": [],
    "RootRedirect": "PATH_TO_REDIRECTION",
    "Writable": false
  },

currently the configuration doesn't seem to work well, because

	if len(cfg.Gateway.RootRedirect) > 0 {
		opts = append(opts, corehttp.RedirectOption("", cfg.Gateway.RootRedirect))
	}

and when path = "" Handler for redirection will be set to "//"

	return func(n *core.IpfsNode, _ net.Listener, mux *http.ServeMux) (*http.ServeMux, error) {
		mux.Handle("/"+path+"/", handler)
		return mux, nil
	}

in local

curl localhost:5001 # returns 404

fix

as I fixed in the diff, handle correctly the case for root redirection.

[MasashiSalvador57f]

wow, I need to fix it for GitCop... :)

[Stebalien]

I'll merge this after the release. Thanks!