fix(mock): matching the complete header list when using fetch

[panva]

Fixes the ability to match a mock on the complete header when using fetch. This ability was introduced in v4.16.0, specifically #1275.

The issue is in the fact that when using fetch the matcher argument is not a plain object but rather HeadersList.

[codecov-commenter]

Codecov Report

Merging #1301 (a925816) into main (a15bef3) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1301      +/-   ##
==========================================
+ Coverage   94.14%   94.16%   +0.01%     
==========================================
  Files          44       45       +1     
  Lines        4100     4112      +12     
==========================================
+ Hits         3860     3872      +12     
  Misses        240      240              

Impacted Files	Coverage Δ
lib/mock/mock-utils.js	100.00% <100.00%> (ø)
lib/fetch/response.js	84.86% <0.00%> (-7.56%)	⬇️
lib/pool-base.js	96.77% <0.00%> (-0.98%)	⬇️
lib/fetch/body.js	98.33% <0.00%> (-0.84%)	⬇️
lib/client.js	97.93% <0.00%> (-0.11%)	⬇️
lib/agent.js	100.00% <0.00%> (ø)
lib/proxy-agent.js	100.00% <0.00%> (ø)
lib/fetch/headers.js	100.00% <0.00%> (ø)
lib/fetch/constants.js	100.00% <0.00%> (ø)
lib/dispatcher-base.js	98.71% <0.00%> (ø)
... and 5 more

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[mcollina]

lgtm