Slightly improve test case

Author: irvind

test/test_http11.py

@@ -132,13 +132,15 @@ def test_iterable_header(self):
             ('User-Agent', 'hyper'), 
             ('Custom-field', 'test'),
             ('Custom-field2', 'test'),
+            ('Custom-field', 'test2'),
         ))
 
         expected = (
             b"GET /get HTTP/1.1\r\n"
             b"User-Agent: hyper\r\n"
             b"Custom-field: test\r\n"
             b"Custom-field2: test\r\n"
+            b"Custom-field: test2\r\n"
             b"connection: Upgrade, HTTP2-Settings\r\n"
             b"upgrade: h2c\r\n"
             b"HTTP2-Settings: AAQAAP//\r\n"