* Treat 304 (Not Modified) requests as valid.

* Add tests for 301-to-200 links, 301-to-404 links, and 500 links.
This helps to test redirections and the previously-added
response.status() checking for non-success status codes in check_url().

* Make names for HTTP mock paths unique, to avoid weird behavior. It
seems like mocks with the same path can potentially bleed between
tests, so you may end up with an unexpected response which causes the
test to sometimes pass and sometimes fail.

* Fix Clippy warnings about String::from(format!()).