fix: readable.setEncoding

[ronag]

Fixes: https://github.com/nodejs/undici/issues/1125#issuecomment-987724800

[ronag]

@koh110 Would you mind fixing tests?

[codecov-commenter]

Codecov Report

Merging #1129 (a0a0f72) into main (ebea0f7) will increase coverage by 2.14%. The diff coverage is 83.33%.

@@            Coverage Diff             @@
##             main    #1129      +/-   ##
==========================================
+ Coverage   94.25%   96.40%   +2.14%     
==========================================
  Files          40       30      -10     
  Lines        3815     2501    -1314     
==========================================
- Hits         3596     2411    -1185     
+ Misses        219       90     -129     

Impacted Files	Coverage Δ
lib/api/readable.js	79.31% <83.33%> (-9.49%)	:arrow_down:
index.js	85.13% <0.00%> (-14.87%)	:arrow_down:
lib/core/util.js	89.14% <0.00%> (-9.31%)	:arrow_down:
lib/handler/redirect.js	87.34% <0.00%> (-7.60%)	:arrow_down:
lib/core/errors.js	91.39% <0.00%> (-3.23%)	:arrow_down:
lib/fetch/formdata.js
lib/fetch/constants.js
lib/fetch/body.js
lib/fetch/util.js
... and 6 more

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[koh110]

@ronag Thanks for your PR. Here is my test code. Would you review it? https://github.com/nodejs/undici/pull/1131

[ronag]

Fixing this is very complicated unfortunately... :/

[koh110]

@ronag @mcollina Hi. I tried to fix this bug. But I'm not sure if I've fixed it perfectly. Would you review it? https://github.com/nodejs/undici/pull/1133