fix: normalize options.headers (again) after onRequest hook

[kricsleo]

resolves #522
(resolves #521)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

@@          Coverage Diff          @@
##             v1     #524   +/-   ##
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  Coverage      ?   82.92%           
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  Files         ?        5           
  Lines         ?      246           
  Branches      ?      122           
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  Hits          ?      204           
  Misses        ?       34           
  Partials      ?        8           

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[danielroe]

@pi0 could we do this at the type level instead? ie disallowing assigning to headers?

[pi0]

@danielroe good idea! I think hooks should accept resolved config type not input. Feel free to drop a PR ❤️