Ensure GET requests do not include a body

[wyattjoh]

When a middleware changes the req.fetchOpts.method to GET, at the moment, it will try to send the body parameter down to fetch which causes an error:

TypeError: Window.fetch: HEAD or GET Request cannot have a body.

This change ensures that requests sent as GET will not include the body.

[wyattjoh]

Note that this further only sends down the parameters used by fetch, noteably not the url. It also ensures that the Content-Type header is not set on the upstream object, and is only sent to fetch if the body is being sent (it is a POST request and not form data).