Fixed grammar in FakePayload exception message.

tests/test_client/test_fakepayload.py

@@ -0,0 +1,11 @@
+from django.test import SimpleTestCase
+from django.test.client import FakePayload
+
+
+class FakePayloadTests(SimpleTestCase):
+    def test_write_after_read(self):
+        payload = FakePayload()
+        payload.read()
+        msg = "Unable to write a payload after it's been read"
+        with self.assertRaisesMessage(ValueError, msg):
+            payload.write(b'abc')