Upgrade to bytes 0.6 (#497)

* Upgrade to bytes 0.6

* Update Cargo.toml

Co-authored-by: Eliza Weisman <eliza@buoyant.io>

* Update tests/h2-support/Cargo.toml

Co-authored-by: Eliza Weisman <eliza@buoyant.io>

Co-authored-by: Eliza Weisman <eliza@buoyant.io>

src/codec/framed_write.rs

@@ -3,7 +3,7 @@ use crate::codec::UserError::*;
 use crate::frame::{self, Frame, FrameSize};
 use crate::hpack;
 
-use bytes::{buf::BufMutExt, Buf, BufMut, BytesMut};
+use bytes::{buf::BufMut, Buf, BytesMut};
 use std::pin::Pin;
 use std::task::{Context, Poll};
 use tokio::io::{AsyncRead, AsyncWrite, ReadBuf};