Use forEach in lib/middleware/karma.js

[BenjaminVanRyseghem]

Use forEach instead of for..in as it's better
practices and fix #2671.

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[BenjaminVanRyseghem]

I signed it!

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CLAs look good, thanks!

[dignifiedquire]

Thank you but we can not make this change, .forEach assumes that files is an unmodified array with an intact prototype, but we had issues where that was not the case.

[BenjaminVanRyseghem]

Thanks for the pointer.

I'll see if I can get another workaround 😄