stop using @_env in the controller instance

Actions are processed through `dispatch`, so they should have the
request set on them before any user land code can be executed.  Lets
stop setting _env on the controller, and give access to it through the
`env` method.

actionpack/lib/action_controller/base.rb

@@ -252,7 +252,7 @@ module ActionController
 
     # Define some internal variables that should not be propagated to the view.
     PROTECTED_IVARS = AbstractController::Rendering::DEFAULT_PROTECTED_INSTANCE_VARIABLES + [
-      :@_status, :@_headers, :@_params, :@_env, :@_response, :@_request,
+      :@_status, :@_headers, :@_params, :@_response, :@_request,
       :@_view_runtime, :@_stream, :@_url_options, :@_action_has_layout ]
 
     def _protected_ivars # :nodoc:

actionpack/lib/action_controller/metal.rb

@@ -119,7 +119,7 @@ module ActionController
     attr_internal_writer :env
 
     def env
-      @_env ||= {}
+      @_request.env
     end
 
     # Returns the last part of the controller's name, underscored, without the ending
@@ -215,8 +215,7 @@ module ActionController
 
     def set_request!(request) #:nodoc:
       @_request = request
-      @_env = request.env
-      @_env['action_controller.instance'] = self
+      @_request.controller_instance = self
     end
 
     def to_a #:nodoc:

actionpack/lib/action_dispatch/http/request.rb

@@ -134,6 +134,14 @@ module ActionDispatch
       end
     end
 
+    def controller_instance # :nodoc:
+      env['action_controller.instance'.freeze]
+    end
+
+    def controller_instance=(controller) # :nodoc:
+      env['action_controller.instance'.freeze] = controller
+    end
+
     def show_exceptions? # :nodoc:
       # We're treating `nil` as "unset", and we want the default setting to be
       # `true`.  This logic should be extracted to `env_config` and calculated

actionpack/lib/action_dispatch/testing/integration.rb

@@ -378,7 +378,7 @@ module ActionDispatch
           @html_document = nil
           @url_options = nil
 
-          @controller = session.last_request.env['action_controller.instance']
+          @controller = @request.controller_instance
 
           response.status
         end

guides/source/debugging_rails_applications.md

@@ -502,7 +502,7 @@ current context:
 
 (byebug) instance_variables
 [:@_action_has_layout, :@_routes, :@_headers, :@_status, :@_request,
- :@_response, :@_env, :@_prefixes, :@_lookup_context, :@_action_name,
+ :@_response, :@_prefixes, :@_lookup_context, :@_action_name,
  :@_response_body, :@marked_for_same_origin_verification, :@_config]
 ```
 
@@ -533,7 +533,7 @@ And then ask again for the instance_variables:
 ```
 (byebug) instance_variables
 [:@_action_has_layout, :@_routes, :@_headers, :@_status, :@_request,
- :@_response, :@_env, :@_prefixes, :@_lookup_context, :@_action_name,
+ :@_response, :@_prefixes, :@_lookup_context, :@_action_name,
  :@_response_body, :@marked_for_same_origin_verification, :@_config,
  :@articles]
 ```