Fixed #33546 -- Optimized ResponseHeaders and HttpResponseBase

[kezabelle]

ticket is 33546.

Lets hope CI doesn't disprove me.

Timings, mirrored from ticket, on my local Macbook Air:

	main	patched
ResponseHeaders({})	745 ns	313 ns
ResponseHeaders({'TEST': "1"})	2.17 µs	1.8 µs
ResponseHeaders({'TEST': "1", "TEST2": "2"})	3.51 µs	2.77 µs
ResponseHeaders({"Content-Type": "application/json"})	2.19 µs	1.93 µs
HttpResponseBase()	7.89 µs	4.18 µs
HttpResponseBase(headers={"TEST": "1"})	9.75 µs	6.02 µs
HttpResponseBase(headers={"TEST": "1", "TEST2": "2"})	11 µs	7.09 µs
HttpResponseBase(headers={"Content-Type": "application/json"})	3.71 µs	3.26 µs
HttpResponseBase(content_type="test")	4.48 µs	3.03 µs
HttpResponse()	9.06 µs	5.46 µs
HttpResponse(headers={"TEST": "1"})	11.4 µs	7.58 µs
HttpResponse(headers={"TEST": "1", "TEST2": "2"})	13 µs	8.83 µs
HttpResponse(headers={"Content-Type": "application/json"})	5.22 µs	4.82 µs
HttpResponse(content_type="test")	6.2 µs	4.38 µs

As always, I'd welcome anyone point out flaws, additional timings to check, or counterpoint datapoints. Also scrutinise my logic, as I've been staring at this for too long ;)

[felixxm]

@kezabelle Thanks 👍

I pushed tiny edits.