make the default environment have actual defaults

instead of deleting keys on every instantiation, create defaults we
actually use.  eventually we can pass an environment in to the request,
and create a new req / res object on each call.

actionpack/lib/action_controller/test_case.rb

@@ -143,6 +143,9 @@ def assert_template(options = {}, message = nil)
   end
 
   class TestRequest < ActionDispatch::TestRequest #:nodoc:
+    DEFAULT_ENV = ActionDispatch::TestRequest::DEFAULT_ENV.dup
+    DEFAULT_ENV.delete 'PATH_INFO'
+
     def initialize(env = {})
       super
 
@@ -207,6 +210,12 @@ def recycle!
       cookie_jar.update(@set_cookies)
       cookie_jar.recycle!
     end
+
+    private
+
+    def default_env
+      DEFAULT_ENV
+    end
   end
 
   class TestResponse < ActionDispatch::TestResponse
@@ -518,8 +527,6 @@ def setup_controller_request_and_response
           @controller ||= klass.new rescue nil
         end
 
-        @request.env.delete('PATH_INFO')
-
         if defined?(@controller) && @controller
           @controller.request = @request
           @controller.params = {}

actionpack/lib/action_dispatch/testing/test_request.rb

@@ -12,7 +12,7 @@ def self.new(env = {})
 
     def initialize(env = {})
       env = Rails.application.env_config.merge(env) if defined?(Rails.application) && Rails.application
-      super(DEFAULT_ENV.merge(env))
+      super(default_env.merge(env))
 
       self.host        = 'test.host'
       self.remote_addr = '0.0.0.0'
@@ -69,5 +69,11 @@ def accept=(mime_types)
     def cookies
       @cookies ||= {}.with_indifferent_access
     end
+
+    private
+
+    def default_env
+      DEFAULT_ENV
+    end
   end
 end