fix: incorrect host when running tests
fix: incorrect host when running tests

As noted in https://github.com/rust-lang/crates.io/issues/493#issuecomment-282514820,
the recorded `http-data` in `src/tests` expects "http://alexcrichton-test.s3.amazonaws.com".
But, as the `S3_REGION` env var is loaded as `Some("")`,
it matches wrong leading to wrong host "http://alexcrichton-test-.s3.amazonaws.com".
This fixes it by handling match when region is empty.
steveklabnik/crates.io,rust-lang/crates.io,steveklabnik/crates.io,rust-lang/crates.io,rust-lang/crates.io,steveklabnik/crates.io,steveklabnik/crates.io,rust-lang/crates.io