Update year in LICENSE and README
Pass `secret` to Rack::Session::Cookie in specs

To avoid a security warning.
Fix spec expectation

In latests RSpec a default declaration expects it to invoke once and
only once.
Add Content-Type to Rack::Builder.app in specs

Just a formality.
Explicitly set Omniauth test mode in specs

And turn it off whenever we are using rack-test to test actuall http
requests (`get '...'`).
Fix request_phase specs

Latest rack-test works a little bit different than previously, so
change accordingly.