Move Transfer-Encoding chunked test to Action Pack

While working on [another PR][1], I found that removing the
Transfer-Encoding conditionals did not result in any failing tests in
Action Pack. This was surprising to me until I found that there was a
test for this behavior in Railties. However, nothing about the test
really depends on having a full Rails application or the Railties test
suite.

This commit moves the test into Action Pack to simplify/speedup the test
(no need to build a full app) as well as keeping the test closer to the
actual behavior being tested.

[1]: 0c334b4

Author: skipkayhil

actionpack/test/controller/chunked_test.rb

@@ -0,0 +1,32 @@
+# frozen_string_literal: true
+
+require "stringio"
+require "abstract_unit"
+
+class ChunkedTest < ActionDispatch::IntegrationTest
+  class ChunkedController < ApplicationController
+    def chunk
+      render json: {
+        raw_post: request.raw_post,
+        content_length: request.content_length
+      }
+    end
+  end
+
+  # The TestInput class prevents Rack::MockRequest from adding a Content-Length when the method `size` is defined
+  class TestInput < StringIO
+    undef_method :size
+  end
+
+  test "parses request raw_post correctly when request has Transfer-Encoding header without a Content-Length value" do
+    @app = self.class.build_app
+    @app.routes.draw do
+      post "chunked", to: ChunkedController.action(:chunk)
+    end
+
+    post "/chunked", params: TestInput.new("foo=bar"), headers: { "Transfer-Encoding" => "gzip, chunked;foo=bar" }
+
+    assert_equal 7, response.parsed_body["content_length"]
+    assert_equal "foo=bar", response.parsed_body["raw_post"]
+  end
+end