Passing redirectPath a function is broken on oauth2

https://github.com/bnoguchi/everyauth/blob/master/lib/modules/oauth2.js#L305-L310

Does not return this, so the chain is killed. I don't think this works either - this block is confusing and undocumented.

Expected behavior:
- Return `this` so you can keep chaining
- redirectPath function gets passed `req` and `res` arguments, and returns a string
