refactor: Get rid of header_utils.ts

[bgoscinski]

I noticed that most of the functions in header_utils.ts were not used and that setting header can be simplified by coercing init.headers to Headers instance.

Feel free to close this if you don't like this refactor :)

[sonarqubecloud]

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[jd1378]

there's many ways to set headers other than the Headers instance, I wanted to support them all, that's why the file exists.

[jd1378]

are you sure it handles them all correctly ?

[jd1378]

yeah okay I guess this is fine as well, thanks for letting me know I could do this

[bgoscinski]

Yes, I think it handles all the ways because we delegate to Headers constructor that accepts optional HeadersInit. RequestInit's headers property is declared also as HeadersInit so it's all compatible as far as I can tell :)