fix[tracing]: fetch headers

[HazAT]

Fixes #2685

The cases I tested

  await fetch('/?1', { headers: {'content-type': 'application/json'} });
  await fetch('/?2', { headers: new Headers({ 'content-type': 'application/json' }) });
  await fetch('/?3', { headers: [['content-type', 'application/json']] });
  await fetch('/?4');

@AbhiPrasad We need to add tests for this in @sentry/tracing

[getsentry-bot]

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[Janpot]

@HazAT Did you test

await fetch(new Request('/?1', { headers: {'content-type': 'application/json'} }));

?