This commit fixes `cargo build` and `cargo test`.
Method lookup on traits seems to have changed to force
`impl TraitName` expressions to be more specific. That means that
`method` will not be found anymore on an object of type `&Trait+Send`,
unless you provide an `impl Trait+Send`.
Now `NetworkStream` and `HeaderFormat` trait implementations
are done against `* + Send`, which helps the compiler to find the
respective `downcast*` method implementations once again.