pathname in baseURL is ignored for redirectURI

{
  baseUrl: 'https://example.com/basename', 
  callbackURI: '/github/callback',
}

With that config, I get the redirect URI https://example.com/github/callback, even though it should contain the /basename. I explicitly don't want to specify /basename/github/callback as the callbackURI, because that would mount the route to that pathname, which would be available at https://example.com/basename/basename/github/callback (notice the double "basename").

I think the problem lies in the usage of url.resolve:

urlObj.pathname = url.resolve(urlObj.pathname, opts.callbackURI)

url.resolve is meant to resolve relative paths, but the callbackURI starts with a slash, so the pathname is ignored. Joining them with a / (and stripping unnecessary slashes) would be preferable in that case. Since all route mountpoints are supposed to start with a slash, we can probably just always change the behaviour and drop url.resolve entirely.

I provide a PR #19 with the suggested fix.

Start at the urlObj.pathname assignment using url.resolve and compare it with PR #19. Done means a baseUrl pathname such as /basename is preserved in the redirect URI without requiring it in callbackURI or duplicating it in the mounted route.