Enable absolute url transform for Request in FastBoot

[xg-wang]

Overrides #356, fixes #355.

The problem is that node-fetch parses input argument in Request's constructor, so any changes made after that are not taken into account, and the request fails with Only absolute URLs are supported error.
Also, the url property is marked as read-only https://developer.mozilla.org/en-US/docs/Web/API/Request/url, so no changes should be made on that property.