Fix GET JSON integration test request to use method override

When a `GET` request is sent `as: :json` in an integration test the test
should use Rack's method override to change to a post request so the
paramters are included in the postdata. Otherwise it will not encode the
parameters correctly for the integration test.

Because integration test sets up it's own middleware,
`Rack::MethodOverride` needs to be included in the integration tests as
well.

`headers ||= {}` was moved so that headers are never nil. They should
default to a hash.

Fixes #26033

[Eileen M. Uchitelle & Aaron Patterson]

actionpack/lib/action_dispatch/testing/integration.rb

@@ -327,6 +327,12 @@ module ActionDispatch
         # Performs the actual request.
         def process(method, path, params: nil, headers: nil, env: nil, xhr: false, as: nil)
           request_encoder = RequestEncoder.encoder(as)
+          headers ||= {}
+
+          if method == :get && as == :json && params
+            headers['X-Http-Method-Override'] = 'GET'
+            method = :post
+          end
 
           if path =~ %r{://}
             path = build_expanded_path(path, request_encoder) do |location|
@@ -361,7 +367,6 @@ module ActionDispatch
           }
 
           if xhr
-            headers ||= {}
             headers['HTTP_X_REQUESTED_WITH'] = 'XMLHttpRequest'
             headers['HTTP_ACCEPT'] ||= [Mime[:js], Mime[:html], Mime[:xml], 'text/xml', '*/*'].join(', ')
           end

actionpack/test/abstract_unit.rb

@@ -104,6 +104,7 @@ class ActionDispatch::IntegrationTest < ActiveSupport::TestCase
       middleware.use ActionDispatch::Callbacks
       middleware.use ActionDispatch::Cookies
       middleware.use ActionDispatch::Flash
+      middleware.use Rack::MethodOverride
       middleware.use Rack::Head
       yield(middleware) if block_given?
     end

actionpack/test/controller/integration_test.rb

@@ -1238,6 +1238,22 @@ class IntegrationRequestEncodersTest < ActionDispatch::IntegrationTest
     end
   end
 
+  def test_get_request_with_json_uses_method_override_and_sends_a_post_request
+    with_routing do |routes|
+      routes.draw do
+        ActiveSupport::Deprecation.silence do
+          get ':action' => FooController
+        end
+      end
+
+      get '/foos_json', params: { foo: 'heyo' }, as: :json
+
+      assert_equal 'POST', request.method
+      assert_equal 'GET', request.headers['X-Http-Method-Override']
+      assert_equal({ 'foo' => 'heyo' }, response.parsed_body)
+    end
+  end
+
   private
     def post_to_foos(as:)
       with_routing do |routes|