Replace reference to non-standard rack header REQUEST_PATH

This header has never been defined by rack. Replaced with the
standard Rack::Request implementation.

Gemfile.lock

@@ -1,7 +1,7 @@
 PATH
   remote: .
   specs:
-    koi (2.6.7)
+    koi (2.6.8)
       acts-as-taggable-on (>= 7.0.0)
       awesome_nested_set (>= 3.3.0)
       cocoon (>= 1.2.15)

lib/koi/middleware/url_redirect.rb

@@ -9,7 +9,7 @@ def initialize(app)
     def call(env)
       status, headers, body = @app.call(env)
 
-      current_path = env["REQUEST_PATH"]
+      current_path = "#{env["SCRIPT_NAME"]}#{env["PATH_INFO"]}"
 
       if status.to_i == 404 && current_path.exclude?("mini-profiler-resources") && new_path = find_redirect(current_path)
         request = ActionDispatch::Request.new(env)

lib/koi/version.rb

@@ -1,5 +1,5 @@
 # frozen_string_literal: true
 
 module Koi
-  VERSION = "2.6.7"
+  VERSION = "2.6.8"
 end