Fix server actions

[PaulAsjes]

Fixes #175

We previously were copying headers from the request via new Headers(request.headers) which it turns out mangles the required headers for server actions.

This cleans up some types and makes sure we don't mess with the headers.

See this issue for more details.

[moroshko]

@PaulAsjes I'm curious why the return type of withAuth when ensureSignedIn is true was modified to include null?

v1.0.1

const { user } = await withAuth({ ensureSignedIn: true });
// user is User

v1.0.2

const { user } = await withAuth({ ensureSignedIn: true });
// user is User | null

[nicknisi]

I think this part should be able to be reversed since Next will throw an error to redirect if the user isn't signed in, when ensureSignedIn is true. Addressed in #185.