Rely only on methods defined by the Rack body spec while injecting

I cannot be sure that every body can be coerced as an array, the only
thing we can rely on is that all the bodies will respond to each. This
fixes rails#252.

lib/web_console.rb

@@ -10,7 +10,7 @@ module WebConsole
   autoload :Evaluator
   autoload :ExceptionMapper
   autoload :Session
-  autoload :Response
+  autoload :Injector
   autoload :Request
   autoload :WhinyRequest
   autoload :Whitelist

lib/web_console/injector.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+module WebConsole
+  # Injects content into a Rack body.
+  class Injector
+    def initialize(body)
+      @body = "".dup
+
+      body.each { |part| @body << part }
+      body.close if body.respond_to?(:close)
+    end
+
+    def inject(content)
+      if position = @body.rindex("</body>")
+        [ @body.insert(position, content) ]
+      else
+        [ @body << content ]
+      end
+    end
+  end
+end

lib/web_console/middleware.rb

@@ -30,16 +30,14 @@ def call(env)
         status, headers, body = call_app(env)
 
         if (session = Session.from(Thread.current)) && acceptable_content_type?(headers)
-          response = Response.new(body, status, headers)
-          template = Template.new(env, session)
+          headers["X-Web-Console-Session-Id"] = session.id
+          headers["X-Web-Console-Mount-Point"] = mount_point
 
-          response.headers["X-Web-Console-Session-Id"] = session.id
-          response.headers["X-Web-Console-Mount-Point"] = mount_point
-          response.write(template.render("index"))
-          response.finish
-        else
-          [ status, headers, body ]
+          template = Template.new(env, session)
+          body = Injector.new(body).inject(template.render("index"))
         end
+
+        [ status, headers, body ]
       end
     rescue => e
       WebConsole.logger.error("\n#{e.class}: #{e}\n\tfrom #{e.backtrace.join("\n\tfrom ")}")
@@ -64,7 +62,7 @@ def json_response(opts = {})
         headers = { "Content-Type" => "application/json; charset = utf-8" }
         body    = yield.to_json
 
-        Rack::Response.new(body, status, headers).finish
+        [ status, headers, [ body ] ]
       end
 
       def json_response_with_session(id, request, opts = {})

test/web_console/injector_test.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+module WebConsole
+  class InjectorTest < ActiveSupport::TestCase
+    test "closes body if closable" do
+      closed = false
+
+      body = [ "foo" ]
+      body.define_singleton_method(:close) { closed = true }
+
+      assert_equal [ "foobar" ], Injector.new(body).inject("bar")
+      assert closed
+    end
+
+    test "support fancy bodies like Rack::BodyProxy" do
+      closed = false
+      body = Rack::BodyProxy.new([ "foo" ]) { closed = true }
+
+      assert_equal [ "foobar" ], Injector.new(body).inject("bar")
+      assert closed
+    end
+
+    test "support fancy bodies like ActionDispatch::Response::RackBody" do
+      body = ActionDispatch::Response.create(200, {}, [ "foo" ]).to_a.last
+
+      assert_equal [ "foobar" ], Injector.new(body).inject("bar")
+    end
+  end
+end