Determine request url before ActionDispatch::ShowExceptions

Rails overrides the PATH_INFO key here:

https://github.com/rails/rails/blob/0f0630aaaeef2a7f9b57e09906a420b99a4f862f/actionpack/lib/action_dispatch/middleware/show_exceptions.rb#L45

causing ::Rack::Request#url to be incorrect by the time the env reaches
our middleware.