Allow disabling redirect following in actix-test

[mpalmer]

PR Type

Feature

PR Checklist

• [x] Tests for the changes have been added / updated.
• [x] Documentation comments have been added / updated.
• [x] A changelog entry has been made for the appropriate packages.
• [x] Format code with the latest stable rustfmt.
• [ ] (Team) Label with affected crates and semver status.

Overview

If you're testing that redirects are being properly generated, then it's useful to not have the client go off on a wild goose chase of its own.