Improve matching of headers assertions.

The current implementation makes assumptions about the case and format of
headers. Introduce methods to handle headers in a case insensitive manner
and reduce churn when comparing with multi-value headers.

actionpack/test/abstract_unit.rb

@@ -457,6 +457,39 @@ def assert_not_set_cookie_header(expected, header = @response.headers["Set-Cooki
   end
 end
 
+module HeadersAssertions
+  def normalize_headers(headers)
+    headers.transform_keys(&:downcase)
+  end
+
+  def assert_headers(expected, actual = @response.headers)
+    actual = normalize_headers(actual)
+    expected.each do |key, value|
+      assert_equal value, actual[key]
+    end
+  end
+
+  def assert_header(key, value, actual = @response.headers)
+    actual = normalize_headers(actual)
+    assert_equal value, actual[key]
+  end
+
+  def assert_not_header(key, actual = @response.headers)
+    actual = normalize_headers(actual)
+    assert_not_includes(actual, key)
+  end
+
+  # This works for most headers, but not all, e.g. `set-cookie`.
+  def normalized_join_header(header)
+    header.is_a?(Array) ? header.join(",") : header
+  end
+
+  def assert_header_value(expected, header)
+    header = normalized_join_header(header)
+    assert_equal header, expected
+  end
+end
+
 class DrivenByRackTest < ActionDispatch::SystemTestCase
   driven_by :rack_test
 end

actionpack/test/dispatch/response_test.rb

@@ -84,8 +84,8 @@ def test_read_body_during_action
     # the response can be built.
     status, headers, body = @response.to_a
     assert_equal 200, status
-    assert_equal({
-      "Content-Type" => "text/html; charset=utf-8"
+    assert_headers({
+      "content-type" => "text/html; charset=utf-8"
     }, headers)
 
     parts = []
@@ -122,9 +122,8 @@ def test_empty_content_type_returns_nil
 
     status, headers, body = @response.to_a
     assert_equal 200, status
-    assert_equal({
-      "Content-Type" => "text/html; charset=utf-8"
-    }, headers)
+
+    assert_headers({ "content-type" => "text/html; charset=utf-8" }, headers)
 
     parts = []
     body.each { |part| parts << part }
@@ -147,8 +146,8 @@ def test_only_set_charset_still_defaults_to_text_html
 
     status, headers, _ = @response.to_a
     assert_equal 200, status
-    assert_equal({
-      "Content-Type" => "text/html; charset=utf-8"
+    assert_headers({
+      "content-type" => "text/html; charset=utf-8"
     }, headers)
   end
 
@@ -382,13 +381,15 @@ def test_only_set_charset_still_defaults_to_text_html
     assert @response.respond_to?(:method_missing, true)
   end
 
+  include HeadersAssertions
+
   test "can be explicitly destructured into status, headers and an enumerable body" do
     response = ActionDispatch::Response.new(404, { "Content-Type" => "text/plain" }, ["Not Found"])
     response.request = ActionDispatch::Request.empty
     status, headers, body = *response
 
     assert_equal 404, status
-    assert_equal({ "Content-Type" => "text/plain" }, headers)
+    assert_headers({ "content-type" => "text/plain" }, headers)
     assert_equal ["Not Found"], body.each.to_a
   end
 
@@ -407,10 +408,10 @@ def test_only_set_charset_still_defaults_to_text_html
     env = Rack::MockRequest.env_for("/")
 
     _status, headers, _body = app.call(env)
-    assert_nil headers["Content-Length"]
+    assert_not_header "content-length", headers
 
     _status, headers, _body = Rack::ContentLength.new(app).call(env)
-    assert_equal "5", headers["Content-Length"]
+    assert_header "content-length", "5", headers
   end
 end
 
@@ -452,24 +453,26 @@ def setup
     assert_not @response.has_header?("Foo")
   end
 
+  include HeadersAssertions
+
   test "add_header" do
     # Add a value to an existing header
-    assert_equal "1,2", @response.add_header("Foo", "2")
-    assert_equal "1,2", @response.get_header("Foo")
+    assert_header_value "1,2", @response.add_header("Foo", "2")
+    assert_header_value "1,2", @response.get_header("Foo")
 
     # Add nil to an existing header
-    assert_equal "1,2", @response.add_header("Foo", nil)
-    assert_equal "1,2", @response.get_header("Foo")
+    assert_header_value "1,2", @response.add_header("Foo", nil)
+    assert_header_value "1,2", @response.get_header("Foo")
 
     # Add nil to a nonexistent header
     assert_nil @response.add_header("Bar", nil)
     assert_not @response.has_header?("Bar")
     assert_nil @response.get_header("Bar")
 
     # Add a value to a nonexistent header
-    assert_equal "1", @response.add_header("Bar", "1")
+    assert_header_value "1", @response.add_header("Bar", "1")
     assert @response.has_header?("Bar")
-    assert_equal "1", @response.get_header("Bar")
+    assert_header_value "1", @response.get_header("Bar")
   end
 end