Add test for multivalued form-encoded element as a list (#4700)

tests/test_requests.py

@@ -868,6 +868,12 @@ class TestRequests:
         assert r.status_code == 200
         assert r.url == httpbin('get?test=foo&test=baz')
 
+    def test_form_encoded_post_query_multivalued_element(self, httpbin):
+        r = requests.Request(method='POST', url=httpbin('post'),
+                             data=dict(test=['foo', 'baz']))
+        prep = r.prepare()
+        assert prep.body == 'test=foo&test=baz'
+
     def test_different_encodings_dont_break_post(self, httpbin):
         r = requests.post(httpbin('post'),
             data={'stuff': json.dumps({'a': 123})},