diff --git a/actionpack/lib/action_controller/testing/integration.rb b/actionpack/lib/action_controller/testing/integration.rb
index d6991ab4f5..a8e8f16d6c 100644
--- a/actionpack/lib/action_controller/testing/integration.rb
+++ b/actionpack/lib/action_controller/testing/integration.rb
@@ -62,8 +62,8 @@ module ActionController
       # with 'HTTP_' if not already.
       def xml_http_request(request_method, path, parameters = nil, headers = nil)
         headers ||= {}
-        headers['X-Requested-With'] = 'XMLHttpRequest'
-        headers['Accept'] ||= [Mime::JS, Mime::HTML, Mime::XML, 'text/xml', Mime::ALL].join(', ')
+        headers['HTTP_X_REQUESTED_WITH'] = 'XMLHttpRequest'
+        headers['HTTP_ACCEPT'] ||= [Mime::JS, Mime::HTML, Mime::XML, 'text/xml', Mime::ALL].join(', ')
         process(request_method, path, parameters, headers)
       end
       alias xhr :xml_http_request
@@ -228,7 +228,7 @@ module ActionController
 
       private
         # Performs the actual request.
-        def process(method, path, parameters = nil, headers = nil)
+        def process(method, path, parameters = nil, rack_environment = nil)
           if path =~ %r{://}
             location = URI.parse(path)
             https! URI::HTTPS === location if location.scheme
@@ -265,9 +265,7 @@ module ActionController
           }
           env = Rack::MockRequest.env_for(path, opts)
 
-          (headers || {}).each do |key, value|
-            key = key.to_s.upcase.gsub(/-/, "_")
-            key = "HTTP_#{key}" unless env.has_key?(key) || key =~ /^HTTP_/
+          (rack_environment || {}).each do |key, value|
             env[key] = value
           end
 
diff --git a/actionpack/test/controller/integration_test.rb b/actionpack/test/controller/integration_test.rb
index c616107324..a2b3ad2106 100644
--- a/actionpack/test/controller/integration_test.rb
+++ b/actionpack/test/controller/integration_test.rb
@@ -123,8 +123,8 @@ class SessionTest < Test::Unit::TestCase
   def test_xml_http_request_get
     path = "/index"; params = "blah"; headers = {:location => 'blah'}
     headers_after_xhr = headers.merge(
-      "X-Requested-With" => "XMLHttpRequest",
-      "Accept"           => "text/javascript, text/html, application/xml, text/xml, */*"
+      "HTTP_X_REQUESTED_WITH" => "XMLHttpRequest",
+      "HTTP_ACCEPT"           => "text/javascript, text/html, application/xml, text/xml, */*"
     )
     @session.expects(:process).with(:get,path,params,headers_after_xhr)
     @session.xml_http_request(:get,path,params,headers)
@@ -133,8 +133,8 @@ class SessionTest < Test::Unit::TestCase
   def test_xml_http_request_post
     path = "/index"; params = "blah"; headers = {:location => 'blah'}
     headers_after_xhr = headers.merge(
-      "X-Requested-With" => "XMLHttpRequest",
-      "Accept"           => "text/javascript, text/html, application/xml, text/xml, */*"
+      "HTTP_X_REQUESTED_WITH" => "XMLHttpRequest",
+      "HTTP_ACCEPT"           => "text/javascript, text/html, application/xml, text/xml, */*"
     )
     @session.expects(:process).with(:post,path,params,headers_after_xhr)
     @session.xml_http_request(:post,path,params,headers)
@@ -143,8 +143,8 @@ class SessionTest < Test::Unit::TestCase
   def test_xml_http_request_put
     path = "/index"; params = "blah"; headers = {:location => 'blah'}
     headers_after_xhr = headers.merge(
-      "X-Requested-With" => "XMLHttpRequest",
-      "Accept"           => "text/javascript, text/html, application/xml, text/xml, */*"
+      "HTTP_X_REQUESTED_WITH" => "XMLHttpRequest",
+      "HTTP_ACCEPT"           => "text/javascript, text/html, application/xml, text/xml, */*"
     )
     @session.expects(:process).with(:put,path,params,headers_after_xhr)
     @session.xml_http_request(:put,path,params,headers)
@@ -153,8 +153,8 @@ class SessionTest < Test::Unit::TestCase
   def test_xml_http_request_delete
     path = "/index"; params = "blah"; headers = {:location => 'blah'}
     headers_after_xhr = headers.merge(
-      "X-Requested-With" => "XMLHttpRequest",
-      "Accept"           => "text/javascript, text/html, application/xml, text/xml, */*"
+      "HTTP_X_REQUESTED_WITH" => "XMLHttpRequest",
+      "HTTP_ACCEPT"           => "text/javascript, text/html, application/xml, text/xml, */*"
     )
     @session.expects(:process).with(:delete,path,params,headers_after_xhr)
     @session.xml_http_request(:delete,path,params,headers)
@@ -163,17 +163,17 @@ class SessionTest < Test::Unit::TestCase
   def test_xml_http_request_head
     path = "/index"; params = "blah"; headers = {:location => 'blah'}
     headers_after_xhr = headers.merge(
-      "X-Requested-With" => "XMLHttpRequest",
-      "Accept"           => "text/javascript, text/html, application/xml, text/xml, */*"
+      "HTTP_X_REQUESTED_WITH" => "XMLHttpRequest",
+      "HTTP_ACCEPT"           => "text/javascript, text/html, application/xml, text/xml, */*"
     )
     @session.expects(:process).with(:head,path,params,headers_after_xhr)
     @session.xml_http_request(:head,path,params,headers)
   end
 
   def test_xml_http_request_override_accept
-    path = "/index"; params = "blah"; headers = {:location => 'blah', "Accept" => "application/xml"}
+    path = "/index"; params = "blah"; headers = {:location => 'blah', "HTTP_ACCEPT" => "application/xml"}
     headers_after_xhr = headers.merge(
-      "X-Requested-With" => "XMLHttpRequest"
+      "HTTP_X_REQUESTED_WITH" => "XMLHttpRequest"
     )
     @session.expects(:process).with(:post,path,params,headers_after_xhr)
     @session.xml_http_request(:post,path,params,headers)