fix #987: srcset not working for source element

[gnijuohz]

This should fix #987.

There is a test for img element's srcset already, I'm not sure whether adding a very similar test is the right way to do it.

[codecov-io]

Codecov Report

Merging #992 into master will decrease coverage by 1.12%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master     #992      +/-   ##
==========================================
- Coverage   89.46%   88.34%   -1.13%     
==========================================
  Files          68       68              
  Lines        3341     3251      -90     
==========================================
- Hits         2989     2872     -117     
- Misses        352      379      +27

Impacted Files	Coverage Δ
src/assets/HTMLAsset.js	85.1% <87.5%> (+1.23%)	⬆️
src/visitors/fs.js	57.63% <0%> (-22.23%)	⬇️
src/visitors/dependencies.js	76.62% <0%> (-9.1%)	⬇️
src/assets/CSSAsset.js	81.73% <0%> (-4.35%)	⬇️
src/visitors/globals.js	92.3% <0%> (-3.85%)	⬇️
src/transforms/babel.js	87.37% <0%> (-2.43%)	⬇️
src/WorkerFarm.js	91.93% <0%> (-1.62%)	⬇️
src/Logger.js	96.55% <0%> (-1.15%)	⬇️
src/utils/getTargetEngines.js	95.5% <0%> (-1.13%)	⬇️
src/assets/JSAsset.js	90.83% <0%> (-0.77%)	⬇️
... and 8 more

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[davidnagli]

👍

[VladimirVaivada]

🐛 I am sorry but issue #987 had not been fixed yet, your test.html does not work with parcel-bundler@1.6.2. At least on my machine... Sorry my English )))

[davidnagli]

@VladimirVaivada This PR was only merged a few days ago, we didn't release these changes yet.

[VladimirVaivada]

👍 Thank you very much! My bad 😊