message: perf(h2): forward Buf::bytes_vectored to SendBuf
diff: @@ -7,6 +7,7 @@ use http::header::{
  use http::HeaderMap;
  use pin_project::pin_project;
  use std::error::Error as StdError;
  +use std::io::IoSlice;
  use super::DecodedLength;
  use crate::body::HttpBody;
  @@ -262,4 +263,8 @@ impl<B: Buf> Buf for SendBuf<B> {
  b.advance(cnt)
  }
  }
  +
  + fn bytes_vectored<'a>(&'a self, dst: &mut [IoSlice<'a>]) -> usize {
  + self.0.as_ref().map(|b| b.bytes_vectored(dst)).unwrap_or(0)
  + }
  }
  