fetch should not require absolute URLs

Currently, `fetch(/test.json')` will fail with in exception that the host cannot be determined.

Start by reproducing the fetch('/test.json') case and trace the fetch entry point to where the host-cannot-be-determined exception is raised. Inspect the existing fetch-related tests, if present, and verify that a relative URL no longer fails because it lacks an absolute host.

Written by the indexing model from the issue text.