Fixed grammar in FakePayload exception message.

django/test/client.py

@@ -76,7 +76,7 @@ class FakePayload:
 
     def write(self, content):
         if self.read_started:
-            raise ValueError("Unable to write a payload after he's been read")
+            raise ValueError("Unable to write a payload after it's been read")
         content = force_bytes(content)
         self.__content.write(content)
         self.__len += len(content)

tests/test_client/test_fakepayload.py

+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')