Do not recode JPEG files in imagepipelines

[drs-11]

This patch completes the last few suggestions on the pull request #3689.

Fixes #3055, closes #3689

[codecov]

Codecov Report

Merging #4753 (bbe24d7) into master (77cd511) will increase coverage by 0.19%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4753      +/-   ##
==========================================
+ Coverage   88.67%   88.87%   +0.19%     
==========================================
  Files         162      162              
  Lines       11013    11026      +13     
  Branches     1802     1808       +6     
==========================================
+ Hits         9766     9799      +33     
+ Misses        966      947      -19     
+ Partials      281      280       -1     

Impacted Files	Coverage Δ
scrapy/pipelines/images.py	97.05% <100.00%> (+1.93%)	⬆️
scrapy/robotstxt.py	97.53% <0.00%> (+22.22%)	⬆️

[Gallaecio]

Nice take over!

The coverage data shows an issue, though: the tests only cover the change in convert_image, the change in get_images, which is the most significant, is not currently covered by tests for scenarios where the new parameter is missing.

[drs-11]

Hey @Gallaecio, I think the test cases cover get_images now. Can you review this?

[Gallaecio]

Awesome!

[drs-11]

Ummm @kmike you requested some changes in the previous PR. This looks good to go now?

[kmike]

The implementation looks good to me overall @drs-11, thanks for picking it up! I haven't really checked the tests, but rely on you all to get them right :) +1 to merge after addressing a couple of comments about warning messages.