Fix ExpatError since dicts don't have a stable order

rtorrent expects the CONTENT_LENGTH header to come directly after the header length (see https://github.com/rakshasa/rtorrent/blob/d396c8e058d7665f7bcf85fc79f44030e5269480/src/rpc/scgi_task.cc#L143 ). However, as Python's dictionaries are unordered, the SCGI header may appear first, causing the ExpatError. This commit fixes the issue by using a list of tuples instead of a dict.