Allow overriding of all headers from passed environment hash

Allow REMOTE_ADDR, HTTP_HOST and HTTP_USER_AGENT to be overridden from
the environment passed into `ActionDispatch::TestRequest.new`.

Fixes #11590

actionpack/CHANGELOG.md

@@ -1,3 +1,10 @@
+*   Allow REMOTE_ADDR, HTTP_HOST and HTTP_USER_AGENT to be overridden from
+    the environment passed into `ActionDispatch::TestRequest.new`.
+
+    Fixes #11590
+
+    *Andrew White*
+
 *   Fix an issue where Journey was failing to clear the named routes hash when the
     routes were reloaded and since it doesn't overwrite existing routes then if a
     route changed but wasn't renamed it kept the old definition. This was being

actionpack/lib/action_dispatch/testing/test_request.rb

@@ -3,7 +3,11 @@
 
 module ActionDispatch
   class TestRequest < Request
-    DEFAULT_ENV = Rack::MockRequest.env_for('/')
+    DEFAULT_ENV = Rack::MockRequest.env_for('/',
+      'HTTP_HOST'       => 'test.host',
+      'REMOTE_ADDR'     => '0.0.0.0',
+      'HTTP_USER_AGENT' => 'Rails Testing'
+    )
 
     def self.new(env = {})
       super
@@ -12,10 +16,6 @@ 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))
-
-      self.host        = 'test.host'
-      self.remote_addr = '0.0.0.0'
-      self.user_agent  = 'Rails Testing'
     end
 
     def request_method=(method)

actionpack/test/dispatch/test_request_test.rb

@@ -62,6 +62,36 @@ class TestRequestTest < ActiveSupport::TestCase
     assert_equal false, req.env.empty?
   end
 
+  test "default remote address is 0.0.0.0" do
+    req = ActionDispatch::TestRequest.new
+    assert_equal '0.0.0.0', req.remote_addr
+  end
+
+  test "allows remote address to be overridden" do
+    req = ActionDispatch::TestRequest.new('REMOTE_ADDR' => '127.0.0.1')
+    assert_equal '127.0.0.1', req.remote_addr
+  end
+
+  test "default host is test.host" do
+    req = ActionDispatch::TestRequest.new
+    assert_equal 'test.host', req.host
+  end
+
+  test "allows host to be overridden" do
+    req = ActionDispatch::TestRequest.new('HTTP_HOST' => 'www.example.com')
+    assert_equal 'www.example.com', req.host
+  end
+
+  test "default user agent is 'Rails Testing'" do
+    req = ActionDispatch::TestRequest.new
+    assert_equal 'Rails Testing', req.user_agent
+  end
+
+  test "allows user agent to be overridden" do
+    req = ActionDispatch::TestRequest.new('HTTP_USER_AGENT' => 'GoogleBot')
+    assert_equal 'GoogleBot', req.user_agent
+  end
+
   private
     def assert_cookies(expected, cookie_jar)
       assert_equal(expected, cookie_jar.instance_variable_get("@cookies"))