[Twisted-web] Changing the order of the request header lines that get sent.

[richard77 at airmail.cc]

Hello subscribers

I am using scrapy 1.4.0 which makes use of twisted 17.9.0 to send and 
receive HTTP requests and responses 
(https://github.com/scrapy/scrapy/blob/master/scrapy/core/downloader/handlers/http11.py 
ScrapyAgent.download_request()). I would like to influence the order in 
which the request header lines get sent.

With scrapy the request header is always handled as a python dictionary 
and therefore can not be sorted. So I was looking at the twisted source 
code to find the place where it sends the request header and 
discontinues to use a dictionary (where I would be able to modify the 
order at this place).

Unfortunately I couldn't find it till now. Agent.request() gets the 
request headers as a dictionary and then returns 
self._requestWithEndpoint() with the headers dictionary in 
https://github.com/twisted/twisted/blob/trunk/src/twisted/web/client.py 
on line 1655.

_AgentBase._requestWithEndpoint() on line 1419 then seems to send the 
request but where can I change the order of the headers? Hopefully you 
can help me with the problem.