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

1 files changed, 7 insertions, 4 deletions

diff --git a/actionpack/lib/action_dispatch/testing/integration.rb b/actionpack/lib/action_dispatch/testing/integration.rb
index 3008d10992..9be3759556 100644
--- a/actionpack/lib/action_dispatch/testing/integration.rb
+++ b/actionpack/lib/action_dispatch/testing/integration.rb
@@ -366,14 +366,17 @@ module ActionDispatch
             "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)