Checking if class exists

[ptmartins]

To check if there is no class with that name we should test against an index of (-1), i.e. , the class doesn't exist so we can add the new class. Otherwise it could keep adding the same class, since className index will always be different from 0.

https://github.com/viljamis/responsive-nav.js/blob/87f62468fc4e3354043d3337affb6b274db1aaac/client/src/helpers.js#L113