Add a test for receiving duplicate headers

tests/test_downloader_handlers.py

@@ -214,6 +214,12 @@ class LargeChunkedFileResource(resource.Resource):
         return server.NOT_DONE_YET
 
 
+class DuplicateHeaderResource(resource.Resource):
+    def render(self, request):
+        request.responseHeaders.setRawHeaders(b"Set-Cookie", [b"a=b", b"c=d"])
+        return b""
+
+
 class HttpTestCase(unittest.TestCase):
     scheme = 'http'
     download_handler_cls: Type = HTTPDownloadHandler
@@ -239,6 +245,7 @@ class HttpTestCase(unittest.TestCase):
         r.putChild(b"contentlength", ContentLengthHeaderResource())
         r.putChild(b"nocontenttype", EmptyContentTypeHeaderResource())
         r.putChild(b"largechunkedfile", LargeChunkedFileResource())
+        r.putChild(b"duplicate-header", DuplicateHeaderResource())
         r.putChild(b"echo", Echo())
         self.site = server.Site(r, timeout=None)
         self.wrapper = WrappingFactory(self.site)
@@ -407,6 +414,16 @@ class HttpTestCase(unittest.TestCase):
             HtmlResponse,
         )
 
+    def test_get_duplicate_header(self):
+        def _test(response):
+            self.assertEqual(
+                response.headers.getlist(b'Set-Cookie'),
+                [b'a=b', b'c=d'],
+            )
+
+        request = Request(self.getURL('duplicate-header'))
+        return self.download_request(request, Spider('foo')).addCallback(_test)
+
 
 class Http10TestCase(HttpTestCase):
     """HTTP 1.0 test case"""