Clarify xhr deprecation message. Don't support kwargs.

Prevent hitting integration tests users with two deprecation warnings by
clarifying how they should upgrade to the request helpers `get` and friends.

It's hard to imagine people having `xhr` calls that use keywords, why not
follow the xhr deprecation message? Why instead insert a middle layer of
migration?

They have to switch to something else anyway, so just show how that looks
and nudge them a bit more.

The code was originally added in #18323,
but then wasn't touched when deprecating xhr in
#18771.

actionpack/lib/action_controller/test_case.rb

@@ -415,8 +415,8 @@ def head(action, *args)
 
       def xml_http_request(*args)
         ActiveSupport::Deprecation.warn(<<-MSG.strip_heredoc)
-          xhr and xml_http_request methods are deprecated in favor of
-          `get :index, xhr: true` and `post :create, xhr: true`
+          `xhr` and `xml_http_request` are deprecated and will be removed in Rails 5.1.
+          Switch to e.g. `post :create, params: { comment: { body: 'Honey bunny' } }, xhr: true`.
         MSG
 
         @request.env['HTTP_X_REQUESTED_WITH'] = 'XMLHttpRequest'

actionpack/lib/action_dispatch/testing/integration.rb

@@ -81,26 +81,14 @@ def head(path, *args)
       #
       # Example:
       #
-      #   xhr :get, '/feed', params: { since: 201501011400 }
-      def xml_http_request(request_method, path, *args)
-        if kwarg_request?(args)
-          params, headers, env = args.first.values_at(:params, :headers, :env)
-        else
-          params = args[0]
-          headers = args[1]
-          env = {}
-
-          if params.present? || headers.present?
-            non_kwarg_request_warning
-          end
-        end
-
+      #   xhr :get, '/feed', since: 201501011400
+      def xml_http_request(request_method, path, parameters = nil, headers_or_env = nil)
         ActiveSupport::Deprecation.warn(<<-MSG.strip_heredoc)
-          xhr and xml_http_request methods are deprecated in favor of
-          `get "/posts", xhr: true` and `post "/posts/1", xhr: true`.
+          `xhr` and `xml_http_request` are deprecated and will be removed in Rails 5.1.
+          Switch to e.g. `post comments_path, params: { comment: { body: 'Honey bunny' } }, xhr: true`.
         MSG
 
-        process(request_method, path, params: params, headers: headers, xhr: true)
+        process(request_method, path, params: parameters, headers: headers_or_env, xhr: true)
       end
       alias xhr :xml_http_request