bpo-20504 : in cgi.py, fix bug when a multipart/form-data request has… (#10638)

* bpo-20504 : in cgi.py, fix bug when a multipart/form-data request has no content-length header

* Add Misc/NEWS.d/next file.

* Add rst formatting for NEWS.d/next file

* Reaplce assert by self.assertEqual
diff --git a/Lib/test/test_cgi.py b/Lib/test/test_cgi.py
index 0922555..ab86771 100644
--- a/Lib/test/test_cgi.py
+++ b/Lib/test/test_cgi.py
@@ -352,6 +352,23 @@
         self.assertEqual(fs.list[0].name, 'submit-name')
         self.assertEqual(fs.list[0].value, 'Larry')
 
+    def test_field_storage_multipart_no_content_length(self):
+        fp = BytesIO(b"""--MyBoundary
+Content-Disposition: form-data; name="my-arg"; filename="foo"
+
+Test
+
+--MyBoundary--
+""")
+        env = {
+            "REQUEST_METHOD": "POST",
+            "CONTENT_TYPE": "multipart/form-data; boundary=MyBoundary",
+            "wsgi.input": fp,
+        }
+        fields = cgi.FieldStorage(fp, environ=env)
+
+        self.assertEqual(len(fields["my-arg"].file.read()), 5)
+
     def test_fieldstorage_as_context_manager(self):
         fp = BytesIO(b'x' * 10)
         env = {'REQUEST_METHOD': 'PUT'}