Make :as option also set request format (AC::TestCase)

right now you'd have to specify both :as and :format:

```
post :create, params: { foo: "bar" } as: :json, format: :json
```

actionpack/lib/action_controller/test_case.rb

@@ -498,10 +498,6 @@ def process(action, *args)
 
         parameters ||= {}
 
-        if format
-          parameters[:format] = format
-        end
-
         @html_document = nil
 
         cookies.update(@request.cookies)
@@ -521,6 +517,10 @@ def process(action, *args)
           format ||= as
         end
 
+        if format
+          parameters[:format] = format
+        end
+
         parameters = parameters.symbolize_keys
 
         generated_extras = @routes.generate_extras(parameters.merge(controller: controller_class_name, action: action.to_s))

actionpack/test/controller/test_case_test.rb

@@ -646,6 +646,11 @@ def test_using_as_json_sets_request_content_type_to_json
     assert_equal 2, @request.request_parameters[:num_value]
   end
 
+  def test_using_as_json_sets_format_json
+    post :render_body, params: { bool_value: true, str_value: "string", num_value: 2 }, as: :json
+    assert_equal "json", @request.format
+  end
+
   def test_mutating_content_type_headers_for_plain_text_files_sets_the_header
     @request.headers["Content-Type"] = "text/plain"
     post :render_body, params: { name: "foo.txt" }