Passing Headers.t to RequestInit.make and Request.t to fetch

[tom-sherman]

I'm not sure about how this would be done but it would be great if I could pass Headers.t to RequestInit.make and Request.t to fetch.

This is possible in the fetch API but not possible in the bindings right now.

My current workaround is to have a couple of cast functions. This function is safe for Request as it extends RequestInit so it's safe to downcast it, it's unsafe for Headers though except for the fetch API.

[TheSpyder]

That headers change is the first thing I did in #30 😂
https://github.com/tinymce/rescript-webapi/compare/fetch-newapi

Supplying a Request.t to fetch also makes sense, similar to how I changed the headers, forcing use of the init object doesn't make a lot of sense to me.