feat(body): change Payload::Data to be a Buf

Closes #1508

BREAKING CHANGE: Each payload chunk must implement `Buf`, instead of
  just `AsRef<[u8]>`.

src/client/conn.rs

@@ -626,7 +626,6 @@ impl<T: Send, B: Send> AssertSend for Connection<T, B>
 where
     T: AsyncRead + AsyncWrite + Send + 'static,
     B: Payload + 'static,
-    B::Data: Send + 'static,
 {}
 
 #[doc(hidden)]