fix URL Prefix

[qdongxu]

This PR addresses Issue #2356.

What is being done in this PR?

Type in here a description of the changes in this PR.
The web console cannot be opened after setting a URL prefix in actions/app.go :

	appOnce.Do(func() {
		app = buffalo.New(buffalo.Options{
			Env:         ENV,
			SessionName: "_prefix_test_session",
			Prefix:      "/prefix/",
		})

The error is :

home/index.plush.html: line 5: "rootPath": unknown identifier

What are the main choices made to get to this solution?

Type in here a description of the choices made to get to this solution.

routenamer.go consider's "/" as the root path only, ignoring a URL prefix as a root path.

List the manual test cases you've covered before sending this PR:

1. Add a prefix as above then open http://127.0.0.1:3000/prefix/ in the browser.

[qdongxu]

closed as submitted to the wrong branch.