code: def parse_headers(header_list):
      
      header_string = b""
      for key, values in header_list.items():
          for v in values:
              header_string += \
                  key.encode('utf-8') + b":" + v.encode('utf-8') + b"\r\n"
      return compat.get_httpmessage(header_string)
docs: Convert headers from our serialized dict with lists for keys to a
      HTTPMessage