Include Content-Length in HTTP/1.1 responses

[Gallaecio]

Alternative to #5034 to deal with #5009 without changes in Twisted, based on #5034 (comment)

Fixes #5009, closes #5034, closes #5045

[codecov]

Codecov Report

Merging #5057 (4da7c0a) into master (0e7ee12) will decrease coverage by 0.34%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master    #5057      +/-   ##
==========================================
- Coverage   87.96%   87.62%   -0.35%     
==========================================
  Files         162      162              
  Lines       10289    10297       +8     
  Branches     1499     1501       +2     
==========================================
- Hits         9051     9023      -28     
- Misses        967     1001      +34     
- Partials      271      273       +2     

Impacted Files	Coverage Δ
scrapy/core/downloader/handlers/http11.py	93.68% <88.88%> (-0.17%)	⬇️
scrapy/utils/asyncgen.py	80.00% <0.00%> (-20.00%)	⬇️
scrapy/utils/ssl.py	53.65% <0.00%> (-17.08%)	⬇️
scrapy/utils/test.py	50.00% <0.00%> (-10.94%)	⬇️
scrapy/utils/reactor.py	75.00% <0.00%> (-6.67%)	⬇️
scrapy/extensions/feedexport.py	91.00% <0.00%> (-4.32%)	⬇️
scrapy/core/scraper.py	87.11% <0.00%> (-2.46%)	⬇️
scrapy/crawler.py	87.15% <0.00%> (-1.12%)	⬇️
scrapy/core/downloader/__init__.py	92.48% <0.00%> (+1.50%)	⬆️