fix(fetch): replace instanceof FormData check

[NMinhNguyen]

See #1451 (comment)

I've tested this using esbuild with --minify and util.isFormData still works on a minified FormData class

[codecov-commenter]

Codecov Report

Merging #1457 (2e5814b) into main (324beb6) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1457      +/-   ##
==========================================
+ Coverage   94.42%   94.44%   +0.01%     
==========================================
  Files          49       49              
  Lines        4271     4285      +14     
==========================================
+ Hits         4033     4047      +14     
  Misses        238      238              

Impacted Files	Coverage Δ
lib/fetch/body.js	98.33% <100.00%> (ø)
lib/fetch/formdata.js	96.70% <100.00%> (+0.03%)	⬆️
lib/core/errors.js	100.00% <0.00%> (ø)
lib/api/api-request.js	100.00% <0.00%> (ø)
lib/core/request.js	98.66% <0.00%> (+<0.01%)	⬆️

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[ronag]

I don't have an opinion on this. +/- 0. @mcollina @KhafraDev @Ethan-Arrowood @targos thoughts?

[mcollina]

lgtm

[mcollina]

Could you add a test?

[NMinhNguyen]

Could you add a test?

I'm not sure what sort of test I could add that tests this edge case. Also util.isFormDataLike is not part of the public API. The original PR that was merged didn't have a test either, as I suspect it's quite tricky to test.

[NMinhNguyen]

Added a simple test, let me know if it's sufficient @mcollina: https://github.com/nodejs/undici/compare/213706c71daa48705cf5a4da096ac0f23c8cbefb..2e5814b077e2695eadcc7b9b87e8e8899031e27c

How frequently do you publish new versions btw?

[mcollina]

Once we have some time. At least once a month, ideally every two weeks.