fix: Allow mounting of sub-app on root

[tomhooijenga]

This should fix #279.

Somehow there was a huge difference between app.use('/', subApp) and app.use('/path', subApp).

[talentlessguy]

does it work with other mountpaths if two apps are mounted on same root?

[tomhooijenga]

Do you mean like this?

app.use('/abc', subApp1);
app.use('/abc', subApp2);

This is covered in this test and still works :)

[talentlessguy]

oh nice, then then I'll merge it after CI passes :D

[talentlessguy]

@all-contributors add @tomhooijenga to code

[allcontributors]

@talentlessguy

I've put up a pull request to add @tomhooijenga! 🎉