Fix overwriting repeated headers in HTTP2

[Malkiz223]

This commit fixes the overwriting bug of repeated header values using HTTP2

raw_response = \
"""
HTTP 1.1 200 OK
Set-Cookie: first=1
Set-Cookie: second=2
"""

# Old logic
>>> response.headers.getlist(b'Set-Cookie')
[b'second=2']

# New logic
>>> response.headers.getlist(b'Set-Cookie')
[b'first=1', b'second=2']

[codecov]

Codecov Report

Merging #5777 (59f0980) into master (3252097) will decrease coverage by 0.13%.
The diff coverage is 100.00%.

❗ Current head 59f0980 differs from pull request most recent head 520be4f. Consider uploading reports for the commit 520be4f to get more accurate results

@@            Coverage Diff             @@
##           master    #5777      +/-   ##
==========================================
- Coverage   88.87%   88.74%   -0.14%     
==========================================
  Files         162      162              
  Lines       10980    10978       -2     
  Branches     1796     1624     -172     
==========================================
- Hits         9759     9742      -17     
- Misses        940      953      +13     
- Partials      281      283       +2     

Impacted Files	Coverage Δ
scrapy/core/http2/stream.py	91.32% <100.00%> (-0.58%)	⬇️
scrapy/robotstxt.py	75.30% <0.00%> (-13.59%)	⬇️
scrapy/downloadermiddlewares/cookies.py	95.78% <0.00%> (-2.11%)	⬇️
scrapy/shell.py	67.91% <0.00%> (-0.75%)	⬇️
scrapy/http/response/__init__.py	97.40% <0.00%> (-0.04%)	⬇️
scrapy/http/request/__init__.py	97.75% <0.00%> (-0.03%)	⬇️
scrapy/commands/check.py	71.01% <0.00%> (ø)

[Gallaecio]

Looks good.

How do you feel about including a test for the fix?

[Malkiz223]

Thank you very much for your answer.

I agree with your policy of writing tests, but this turned out to be too complicated for me.
I spent several hours writing the test, but could not make it correct.

[Gallaecio]

Thanks!