Fix Accept header overridden when "xhr: true" in integration test

In integration test when specify the "Accept" header with "xhr: true"
option, the Accept header is overridden with a default xhr Accept
header. The issue only affects HTTP header "Accept" but not CGI variable
"HTTP_ACCEPT".

For example:

    get '/page', headers: { 'Accept' => 'application/json' }, xhr: true

    # This is WRONG! And the response.content_type is also affected.
    # It should be "application/json"
    assert_equal "text/javascript, text/html, ...", request.accept
    assert_equal 'text/html', response.content_type

The issue is in `ActionDispatch::Integration::RequestHelpers`. When
setting "xhr: true" the helper sets a default HTTP_ACCEPT if blank.
But the code doesn't consider supporting both HTTP header style and
CGI variable style.

For detail see this GitHub issue:
https://github.com/rails/rails/issues/25859

actionpack/CHANGELOG.md

+*   Don't override the `Accept` header in integration tests when called with `xhr: true`.
+
+    Fixes #25859.
+
+    *David Chen*
+
 *   Fix 'defaults' option for root route.
 
     A regression from some refactoring for the 5.0 release, this change

actionpack/lib/action_dispatch/testing/integration.rb

@@ -366,14 +366,17 @@ def process(method, path, params: nil, headers: nil, env: nil, xhr: false, as: n
             "HTTP_ACCEPT"    => accept
           }
 
+          wrapped_headers = Http::Headers.from_hash({})
+          wrapped_headers.merge!(headers) if headers
+
           if xhr
-            headers["HTTP_X_REQUESTED_WITH"] = "XMLHttpRequest"
-            headers["HTTP_ACCEPT"] ||= [Mime[:js], Mime[:html], Mime[:xml], "text/xml", "*/*"].join(", ")
+            wrapped_headers["HTTP_X_REQUESTED_WITH"] = "XMLHttpRequest"
+            wrapped_headers["HTTP_ACCEPT"] ||= [Mime[:js], Mime[:html], Mime[:xml], "text/xml", "*/*"].join(", ")
           end
 
           # this modifies the passed request_env directly
-          if headers.present?
-            Http::Headers.from_hash(request_env).merge!(headers)
+          if wrapped_headers.present?
+            Http::Headers.from_hash(request_env).merge!(wrapped_headers)
           end
           if env.present?
             Http::Headers.from_hash(request_env).merge!(env)

actionpack/test/controller/integration_test.rb

@@ -403,6 +403,7 @@ def get
       respond_to do |format|
         format.html { render plain: "OK", status: 200 }
         format.js { render plain: "JS OK", status: 200 }
+        format.json { render json: "JSON OK", status: 200 }
         format.xml { render xml: "<root></root>", status: 200 }
         format.rss { render xml: "<root></root>", status: 200 }
         format.atom { render xml: "<root></root>", status: 200 }
@@ -727,6 +728,18 @@ def test_respect_removal_of_default_headers_by_a_controller_action
     assert_includes @response.headers, "c"
   end
 
+  def test_accept_not_overriden_when_xhr_true
+    with_test_route_set do
+      get "/get", headers: { "Accept" => "application/json" }, xhr: true
+      assert_equal "application/json", request.accept
+      assert_equal "application/json", response.content_type
+
+      get "/get", headers: { "HTTP_ACCEPT" => "application/json" }, xhr: true
+      assert_equal "application/json", request.accept
+      assert_equal "application/json", response.content_type
+    end
+  end
+
   private
     def with_default_headers(headers)
       original = ActionDispatch::Response.default_headers