add tests for multiple headers with same name

tests/test_http_headers.py

@@ -38,6 +38,13 @@ class HeadersTest(unittest.TestCase):
         self.assertEqual(h.getlist('X-Forwarded-For'), [b'ip1', b'ip2'])
         assert h.getlist('X-Forwarded-For') is not hlist
 
+    def test_multivalue_for_one_header(self):
+        h = Headers((("a", "b"), ("a", "c")))
+        self.assertEqual(h["a"], b"c")
+        self.assertEqual(h.get("a"), b"c")
+        self.assertEqual(h.getlist("a"), [b"b", b"c"])
+        assert h.getlist("a") is not ["b", "c"]
+
     def test_encode_utf8(self):
         h = Headers({'key': '\xa3'}, encoding='utf-8')
         key, val = dict(h).popitem()