Do not pass redirect_uri by default

Stripe requires whitelisting specific redirect_uri values.  If you're only going to use the default anyway, it's better not to pass it to avoid conflicts.  

Currently, Stripe allows any value for redirect_uri but ignores it.  In the future this behavior will be deprecated and passing a redirect_uri parameter that does not exactly match the one in the dashboard will cause an error.

Note this commit introduces rspec testing to match the OmniAuth gems.