(fix) Recast Str to Vec<u8> for rust-http changes

This is as a result of https://github.com/chris-morgan/rust-http/pull/164, which has already been accounted for in Iron.

It makes the test less robust (as it tests for Str, not Vec<u8>), but maintains the ergonomics that was already present.
IMO, a future PR should allow for testing of Vec<u8> outside of the current tests for Str, for the sake of ergonomics.