message: feat(http): implement `Default` for `Chunk`
  This allows using `Stream::concat2` with a `hyper::Body`.
diff: @@ -109,6 +109,13 @@ impl fmt::Debug for Chunk {
  }
  }
  +impl Default for Chunk {
  + #[inline]
  + fn default() -> Chunk {
  + Chunk(Inner::Shared(Bytes::new()))
  + }
  +}
  +
  impl IntoIterator for Chunk {
  type Item = u8;
  type IntoIter = <Bytes as IntoIterator>::IntoIter;
  