perf(h2): forward Buf::bytes_vectored to SendBuf

src/proto/h2/mod.rs

@@ -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)
+    }
 }