Fix generators not respecting headers

[EliLichtblau]

import { Elysia } from 'elysia'
import { node } from '@elysiajs/node'

new Elysia({ adapter: node() })
	.get('/', function*({set}) {
		set.headers["content-type"] = "application/jsonl"
		set.headers["content-disposition"] = 'attachment; filename="export.jsonl"'
		for (let i = 0; i <100; i++) {
			yield JSON.stringify({i})+"\n"
		}
	})
	.listen(3000)

This would ignore the set headers because it isn't defined until the first generator.next() call I think. This fixes that bug and adds some test cases for it