run-dev: Discard Transfer-Encoding header from upstream response.

When the upstream provides a chunked response, proxying this header
causes a protocol-level miscommunication.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

tools/run-dev.py

@@ -261,6 +261,7 @@ class BaseHandler(web.RequestHandler):
 
             for header, v in response.headers.get_all():
                 # some header appear multiple times, eg 'Set-Cookie'
+                if header.lower() != "transfer-encoding":
                     self.add_header(header, v)
             if response.body:
                 self.write(response.body)