Support `Headers` in init

[ankon]

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Basically this comment says it:

https://github.com/Azure/fetch-event-source/blob/a0529492576e094374602f24d5e64b3a271b4576/src/fetch.ts#L10

It would be nice to support Headers as well here, as failing requests because of this problem can be quite hard to debug.

[piotr-cz]

My workaround:

const headers = new Headers()

fetchEventSource('/api/sse', {
      headers: Object.fromEntries(headers),
})