Reorder equality so happy path hits Nil#==

For most requests the expectation is that the application will respond
to the request and not set the `x-cascade` header. Since the header will
be `Nil` in that happy path, we should prefer calling `#==` on the
header itself because `String#==` will try to coerce `other` `to_str`,
whereas `Nil#==` will not.

(`String#==` showed up as ~2.5% in a profile, and this eliminates it)

Author: skipkayhil

actionpack/lib/action_dispatch/journey/router.rb

@@ -43,7 +43,7 @@ def serve(req)
 
           _, headers, _ = response = route.app.serve(req)
 
-          if "pass" == headers[Constants::X_CASCADE]
+          if headers[Constants::X_CASCADE] == "pass"
             req.script_name     = script_name
             req.path_info       = path_info
             req.path_parameters = set_params