Fixed regression that tests using format still work

Error only occurred on tests which return no content and use
a renderer without charset (e.g. JSONRenderer)

rest_framework/test.py

@@ -195,7 +195,7 @@ class APIRequestFactory(DjangoRequestFactory):
                 ret = renderer.render(data)
 
             # Coerce text to bytes if required.
-            if isinstance(ret, str):
+            if isinstance(ret, str) and renderer.charset:
                 ret = ret.encode(renderer.charset)
 
         return ret, content_type

tests/test_testing.py

@@ -8,9 +8,9 @@ from django.shortcuts import redirect
 from django.test import TestCase, override_settings
 from django.urls import path
 
-from rest_framework import fields, parsers, serializers
+from rest_framework import fields, parsers, renderers, serializers, status
 from rest_framework.authtoken.models import Token
-from rest_framework.decorators import api_view, parser_classes
+from rest_framework.decorators import api_view, parser_classes, renderer_classes
 from rest_framework.response import Response
 from rest_framework.test import (
     APIClient, APIRequestFactory, URLPatternsTestCase, force_authenticate
@@ -55,6 +55,10 @@ class BasicSerializer(serializers.Serializer):
 def post_json_view(request):
     return Response(request.data)
 
+@api_view(['DELETE'])
+@renderer_classes((renderers.JSONRenderer, ))
+def delete_json_view(request):
+    return Response(status=status.HTTP_204_NO_CONTENT)
 
 @api_view(['POST'])
 def post_view(request):
@@ -69,6 +73,7 @@ urlpatterns = [
     path('redirect-view/', redirect_view),
     path('redirect-view/<int:code>/', redirect_307_308_view),
     path('post-json-view/', post_json_view),
+    path('delete-json-view/', delete_json_view),
     path('post-view/', post_view),
 ]
 
@@ -254,6 +259,11 @@ class TestAPITestClient(TestCase):
         assert response.status_code == 200
         assert response.data == data
 
+    def test_delete_based_on_format(self):
+        response = self.client.delete('/delete-json-view/', format='json')
+        assert response.status_code == status.HTTP_204_NO_CONTENT
+        assert response.data is None
+
 
 class TestAPIRequestFactory(TestCase):
     def test_csrf_exempt_by_default(self):