Be more forgiving with HEROKU_URL envvar

[patcon]

Why not just add the trailing slash if it's not there? Further, why not just allow someone to use the app name slug?

I ask not rhetorically, but because I suspect there might be good reason :)

[treby]

There are no reasons, as far as I know.
I think it's better adding slash if there is no slash, as you say.
Patches welcome :)