Add Content-Encoding header in response flag

[hassaan-naushahi]

Fixes #1988, closes #4025, closes #5290

[wRAR]

Two tests are failing now, one because there is a new warning which it doesn't expect and another one probably because the header is dropped by default?

This PR is also missing the default setting value, docs and tests for the new setting. Some of those already exist in #5290 so it should be possible to copy them (though I haven't checked the test changes there).

[hassaan-naushahi]

Added values to default_settings.py for successful test runs

[wRAR]

You should keep the empty line before CONCURRENT_ITEMS

[codecov]

Codecov Report

Merging #5943 (8e76b5f) into master (70ba3a0) will decrease coverage by 46.12%.
The diff coverage is 23.33%.

❗ Current head 8e76b5f differs from pull request most recent head 32c7da5. Consider uploading reports for the commit 32c7da5 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #5943       +/-   ##
===========================================
- Coverage   88.59%   42.48%   -46.12%     
===========================================
  Files         159      163        +4     
  Lines       11582    11551       -31     
  Branches     1885     1880        -5     
===========================================
- Hits        10261     4907     -5354     
- Misses        994     6267     +5273     
- Partials      327      377       +50     

Files	Coverage Δ
scrapy/settings/default_settings.py	98.80% <100.00%> (ø)
scrapy/downloadermiddlewares/httpcompression.py	37.50% <20.68%> (-52.21%)	⬇️

... and 145 files with indirect coverage changes

[wRAR]

Can you please fix conflicts?

[wRAR]

Can you please always run tests locally before publishing?